Version bump for release 0.1.8 RC 1

This adds the rule “(not F or G) === (F -> G)” to the simplification
rule tableau.

CHANGELOG.md

@@ -1,6 +1,10 @@
 # Change Log
 
-## (unreleased)
+## 0.1.8 RC 1 (2018-04-10)
+
+### Features
+
+* more, advanced simplification rules
 
 ## 0.1.7 (2018-04-08)
 

app/main.cpp

@@ -70,7 +70,7 @@ int main(int argc, char **argv)
 
 	if (version)
 	{
-		std::cout << "anthem version 0.1.7+git" << std::endl;
+		std::cout << "anthem version 0.1.8-rc.1" << std::endl;
 		return EXIT_SUCCESS;
 	}
 

examples/schur-numbers.lp

@@ -3,3 +3,5 @@ covered(I) :- in(I, S).
 
 :- I = 1..n, not covered(I).
 :- in(I, S), in(J, S), in(I + J, S).
+
+#show in/2.

include/anthem/Equality.h

@@ -0,0 +1,417 @@
+#ifndef __ANTHEM__EQUALITY_H
+#define __ANTHEM__EQUALITY_H
+
+#include <anthem/AST.h>
+#include <anthem/ASTUtils.h>
+
+namespace anthem
+{
+namespace ast
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// Equality
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+// TODO: move to separate class
+enum class Tristate
+{
+	True,
+	False,
+	Unknown,
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+Tristate equal(const Formula &lhs, const Formula &rhs);
+Tristate equal(const Term &lhs, const Term &rhs);
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct FormulaEqualityVisitor
+{
+	Tristate visit(const And &and_, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<And>())
+			return Tristate::Unknown;
+
+		const auto &otherAnd = otherFormula.get<And>();
+
+		for (const auto &argument : and_.arguments)
+		{
+			const auto match = std::find_if(
+				otherAnd.arguments.cbegin(), otherAnd.arguments.cend(),
+				[&](const auto &otherArgument)
+				{
+					return equal(argument, otherArgument) == Tristate::True;
+				});
+
+			if (match == otherAnd.arguments.cend())
+				return Tristate::Unknown;
+		}
+
+		for (const auto &otherArgument : otherAnd.arguments)
+		{
+			const auto match = std::find_if(
+				and_.arguments.cbegin(), and_.arguments.cend(),
+				[&](const auto &argument)
+				{
+					return equal(otherArgument, argument) == Tristate::True;
+				});
+
+			if (match == and_.arguments.cend())
+				return Tristate::Unknown;
+		}
+
+		return Tristate::True;
+	}
+
+	Tristate visit(const Biconditional &biconditional, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<Biconditional>())
+			return Tristate::Unknown;
+
+		const auto &otherBiconditional = otherFormula.get<Biconditional>();
+
+		if (equal(biconditional.left, otherBiconditional.left) == Tristate::True
+		    && equal(biconditional.right, otherBiconditional.right) == Tristate::True)
+		{
+			return Tristate::True;
+		}
+
+		if (equal(biconditional.left, otherBiconditional.right) == Tristate::True
+		    && equal(biconditional.right, otherBiconditional.left) == Tristate::True)
+		{
+			return Tristate::True;
+		}
+
+		return Tristate::Unknown;
+	}
+
+	Tristate visit(const Boolean &boolean, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<Boolean>())
+			return Tristate::Unknown;
+
+		const auto &otherBoolean = otherFormula.get<Boolean>();
+
+		return (boolean.value == otherBoolean.value)
+			? Tristate::True
+			: Tristate::False;
+	}
+
+	Tristate visit(const Comparison &comparison, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<Comparison>())
+			return Tristate::Unknown;
+
+		const auto &otherComparison = otherFormula.get<Comparison>();
+
+		if (comparison.operator_ != otherComparison.operator_)
+			return Tristate::Unknown;
+
+		if (equal(comparison.left, otherComparison.left) == Tristate::True
+		    && equal(comparison.right, otherComparison.right) == Tristate::True)
+		{
+			return Tristate::True;
+		}
+
+		// Only = and != are commutative operators, all others don’t need to be checked with exchanged arguments
+		if (comparison.operator_ != Comparison::Operator::Equal
+		    && comparison.operator_ != Comparison::Operator::NotEqual)
+		{
+			return Tristate::Unknown;
+		}
+
+		if (equal(comparison.left, otherComparison.right) == Tristate::True
+		    && equal(comparison.right, otherComparison.left) == Tristate::True)
+		{
+			return Tristate::True;
+		}
+
+		return Tristate::Unknown;
+	}
+
+	Tristate visit(const Exists &, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<Exists>())
+			return Tristate::Unknown;
+
+		// TODO: implement stronger check
+		return Tristate::Unknown;
+	}
+
+	Tristate visit(const ForAll &, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<ForAll>())
+			return Tristate::Unknown;
+
+		// TODO: implement stronger check
+		return Tristate::Unknown;
+	}
+
+	Tristate visit(const Implies &implies, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<Implies>())
+			return Tristate::Unknown;
+
+		const auto &otherImplies = otherFormula.get<Implies>();
+
+		if (equal(implies.antecedent, otherImplies.antecedent) == Tristate::True
+		    && equal(implies.consequent, otherImplies.consequent) == Tristate::True)
+		{
+			return Tristate::True;
+		}
+
+		return Tristate::Unknown;
+	}
+
+	Tristate visit(const In &in, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<In>())
+			return Tristate::Unknown;
+
+		const auto &otherIn = otherFormula.get<In>();
+
+		if (equal(in.element, otherIn.element) == Tristate::True
+		    && equal(in.set, otherIn.set) == Tristate::True)
+		{
+			return Tristate::True;
+		}
+
+		return Tristate::Unknown;
+	}
+
+	Tristate visit(const Not &not_, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<Not>())
+			return Tristate::Unknown;
+
+		const auto &otherNot = otherFormula.get<Not>();
+
+		return equal(not_.argument, otherNot.argument);
+	}
+
+	Tristate visit(const Or &or_, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<Or>())
+			return Tristate::Unknown;
+
+		const auto &otherOr = otherFormula.get<Or>();
+
+		for (const auto &argument : or_.arguments)
+		{
+			const auto match = std::find_if(
+				otherOr.arguments.cbegin(), otherOr.arguments.cend(),
+				[&](const auto &otherArgument)
+				{
+					return equal(argument, otherArgument) == Tristate::True;
+				});
+
+			if (match == otherOr.arguments.cend())
+				return Tristate::Unknown;
+		}
+
+		for (const auto &otherArgument : otherOr.arguments)
+		{
+			const auto match = std::find_if(
+				or_.arguments.cbegin(), or_.arguments.cend(),
+				[&](const auto &argument)
+				{
+					return equal(otherArgument, argument) == Tristate::True;
+				});
+
+			if (match == or_.arguments.cend())
+				return Tristate::Unknown;
+		}
+
+		return Tristate::True;
+	}
+
+	Tristate visit(const Predicate &predicate, const Formula &otherFormula)
+	{
+		if (!otherFormula.is<Predicate>())
+			return Tristate::Unknown;
+
+		const auto &otherPredicate = otherFormula.get<Predicate>();
+
+		if (!matches(predicate, otherPredicate))
+			return Tristate::False;
+
+		assert(predicate.arguments.size() == otherPredicate.arguments.size());
+
+		for (size_t i = 0; i < predicate.arguments.size(); i++)
+			if (equal(predicate.arguments[i], otherPredicate.arguments[i]) != Tristate::True)
+				return Tristate::Unknown;
+
+		return Tristate::True;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct TermEqualityVisitor
+{
+	Tristate visit(const BinaryOperation &binaryOperation, const Term &otherTerm)
+	{
+		if (!otherTerm.is<BinaryOperation>())
+			return Tristate::Unknown;
+
+		const auto &otherBinaryOperation = otherTerm.get<BinaryOperation>();
+
+		if (binaryOperation.operator_ != otherBinaryOperation.operator_)
+			return Tristate::Unknown;
+
+		if (equal(binaryOperation.left, otherBinaryOperation.left) == Tristate::True
+		    && equal(binaryOperation.right, otherBinaryOperation.right) == Tristate::True)
+		{
+			return Tristate::True;
+		}
+
+		// Only + and * are commutative operators, all others don’t need to be checked with exchanged arguments
+		if (binaryOperation.operator_ != BinaryOperation::Operator::Plus
+		    && binaryOperation.operator_ != BinaryOperation::Operator::Multiplication)
+		{
+			return Tristate::Unknown;
+		}
+
+		if (equal(binaryOperation.left, binaryOperation.right) == Tristate::True
+		    && equal(binaryOperation.right, binaryOperation.left) == Tristate::True)
+		{
+			return Tristate::True;
+		}
+
+		return Tristate::Unknown;
+	}
+
+	Tristate visit(const Boolean &boolean, const Term &otherTerm)
+	{
+		if (!otherTerm.is<Boolean>())
+			return Tristate::Unknown;
+
+		const auto &otherBoolean = otherTerm.get<Boolean>();
+
+		return (boolean.value == otherBoolean.value)
+			? Tristate::True
+			: Tristate::False;
+	}
+
+	Tristate visit(const Constant &constant, const Term &otherTerm)
+	{
+		if (!otherTerm.is<Constant>())
+			return Tristate::Unknown;
+
+		const auto &otherConstant = otherTerm.get<Constant>();
+
+		return (constant.name == otherConstant.name)
+			? Tristate::True
+			: Tristate::False;
+	}
+
+	Tristate visit(const Function &function, const Term &otherTerm)
+	{
+		if (!otherTerm.is<Function>())
+			return Tristate::Unknown;
+
+		const auto &otherFunction = otherTerm.get<Function>();
+
+		if (function.name != otherFunction.name)
+			return Tristate::False;
+
+		if (function.arguments.size() != otherFunction.arguments.size())
+			return Tristate::False;
+
+		for (size_t i = 0; i < function.arguments.size(); i++)
+			if (equal(function.arguments[i], otherFunction.arguments[i]) != Tristate::True)
+				return Tristate::Unknown;
+
+		return Tristate::True;
+	}
+
+	Tristate visit(const Integer &integer, const Term &otherTerm)
+	{
+		if (!otherTerm.is<Integer>())
+			return Tristate::Unknown;
+
+		const auto &otherInteger = otherTerm.get<Integer>();
+
+		return (integer.value == otherInteger.value)
+			? Tristate::True
+			: Tristate::False;
+	}
+
+	Tristate visit(const Interval &interval, const Term &otherTerm)
+	{
+		if (!otherTerm.is<Interval>())
+			return Tristate::Unknown;
+
+		const auto &otherInterval = otherTerm.get<Interval>();
+
+		if (equal(interval.from, otherInterval.from) != Tristate::True)
+			return Tristate::Unknown;
+
+		if (equal(interval.to, otherInterval.to) != Tristate::True)
+			return Tristate::Unknown;
+
+		return Tristate::True;
+	}
+
+	Tristate visit(const SpecialInteger &specialInteger, const Term &otherTerm)
+	{
+		if (!otherTerm.is<SpecialInteger>())
+			return Tristate::Unknown;
+
+		const auto &otherSpecialInteger = otherTerm.get<SpecialInteger>();
+
+		return (specialInteger.type == otherSpecialInteger.type)
+			? Tristate::True
+			: Tristate::False;
+	}
+
+	Tristate visit(const String &string, const Term &otherTerm)
+	{
+		if (!otherTerm.is<String>())
+			return Tristate::Unknown;
+
+		const auto &otherString = otherTerm.get<String>();
+
+		return (string.text == otherString.text)
+			? Tristate::True
+			: Tristate::False;
+	}
+
+	Tristate visit(const Variable &variable, const Term &otherTerm)
+	{
+		if (!otherTerm.is<Variable>())
+			return Tristate::Unknown;
+
+		const auto &otherVariable = otherTerm.get<Variable>();
+
+		return (variable.declaration == otherVariable.declaration)
+			? Tristate::True
+			: Tristate::False;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+Tristate equal(const Formula &lhs, const Formula &rhs)
+{
+	return lhs.accept(FormulaEqualityVisitor(), rhs);
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+Tristate equal(const Term &lhs, const Term &rhs)
+{
+	return lhs.accept(TermEqualityVisitor(), rhs);
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
+
+#endif

include/anthem/Simplification.h

@@ -12,6 +12,14 @@ namespace anthem
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+enum class SimplificationResult
+{
+	Simplified,
+	Unchanged,
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
 void simplify(ast::Formula &formula);
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////

include/anthem/SimplificationVisitors.h

@@ -0,0 +1,198 @@
+#ifndef __ANTHEM__SIMPLIFICATION_VISITORS_H
+#define __ANTHEM__SIMPLIFICATION_VISITORS_H
+
+#include <anthem/AST.h>
+#include <anthem/Simplification.h>
+
+namespace anthem
+{
+namespace ast
+{
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+//
+// Simplification Visitor
+//
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+template<class T>
+struct FormulaSimplificationVisitor
+{
+	template <class... Arguments>
+	SimplificationResult visit(And &and_, Formula &formula, Arguments &&... arguments)
+	{
+		for (auto &argument : and_.arguments)
+			if (argument.accept(*this, argument, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+				return SimplificationResult::Simplified;
+
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Biconditional &biconditional, Formula &formula, Arguments &&... arguments)
+	{
+		if (biconditional.left.accept(*this, biconditional.left, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		if (biconditional.right.accept(*this, biconditional.right, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Boolean &, Formula &formula, Arguments &&... arguments)
+	{
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Comparison &, Formula &formula, Arguments &&... arguments)
+	{
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Exists &exists, Formula &formula, Arguments &&... arguments)
+	{
+		if (exists.argument.accept(*this, exists.argument, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(ForAll &forAll, Formula &formula, Arguments &&... arguments)
+	{
+		if (forAll.argument.accept(*this, forAll.argument, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Implies &implies, Formula &formula, Arguments &&... arguments)
+	{
+		if (implies.antecedent.accept(*this, implies.antecedent, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		if (implies.consequent.accept(*this, implies.consequent, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(In &, Formula &formula, Arguments &&... arguments)
+	{
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Not &not_, Formula &formula, Arguments &&... arguments)
+	{
+		if (not_.argument.accept(*this, not_.argument, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Or &or_, Formula &formula, Arguments &&... arguments)
+	{
+		for (auto &argument : or_.arguments)
+			if (argument.accept(*this, argument, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+				return SimplificationResult::Simplified;
+
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Predicate &, Formula &formula, Arguments &&... arguments)
+	{
+		return T::accept(formula, std::forward<Arguments>(arguments)...);
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+template<class T, class SimplificationResult = void>
+struct TermSimplificationVisitor
+{
+	template <class... Arguments>
+	SimplificationResult visit(BinaryOperation &binaryOperation, Term &term, Arguments &&... arguments)
+	{
+		if (binaryOperation.left.accept(*this, binaryOperation.left, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		if (binaryOperation.right.accept(*this, binaryOperation.right, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		return T::accept(term, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Boolean &, Term &term, Arguments &&... arguments)
+	{
+		return T::accept(term, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Constant &, Term &term, Arguments &&... arguments)
+	{
+		return T::accept(term, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Function &function, Term &term, Arguments &&... arguments)
+	{
+		for (auto &argument : function.arguments)
+			if (argument.accept(*this, argument, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+				return SimplificationResult::Simplified;
+
+		return T::accept(term, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Integer &, Term &term, Arguments &&... arguments)
+	{
+		return T::accept(term, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Interval &interval, Term &term, Arguments &&... arguments)
+	{
+		if (interval.from.accept(*this, interval.from, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		if (interval.to.accept(*this, interval.to, std::forward<Arguments>(arguments)...) == SimplificationResult::Simplified)
+			return SimplificationResult::Simplified;
+
+		return T::accept(term, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(SpecialInteger &, Term &term, Arguments &&... arguments)
+	{
+		return T::accept(term, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(String &, Term &term, Arguments &&... arguments)
+	{
+		return T::accept(term, std::forward<Arguments>(arguments)...);
+	}
+
+	template <class... Arguments>
+	SimplificationResult visit(Variable &, Term &term, Arguments &&... arguments)
+	{
+		return T::accept(term, std::forward<Arguments>(arguments)...);
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+}
+}
+
+#endif

src/anthem/Simplification.cpp

@@ -3,7 +3,9 @@
 #include <optional>
 
 #include <anthem/ASTCopy.h>
-#include <anthem/ASTVisitors.h>
+#include <anthem/Equality.h>
+#include <anthem/output/AST.h>
+#include <anthem/SimplificationVisitors.h>
 
 namespace anthem
 {
@@ -97,18 +99,65 @@ struct ReplaceVariableInFormulaVisitor : public ast::RecursiveFormulaVisitor<Rep
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-// Simplifies exists statements by using the equivalence “exists X (X = t and F(X))” == “F(t)”
-// The exists statement has to be of the form “exists <variables> <conjunction>”
-void simplify(ast::Exists &exists, ast::Formula &formula)
+template<class SimplificationRule>
+SimplificationResult simplify(ast::Formula &formula)
 {
-	// Simplify formulas like “exists X (X = Y)” to “#true”
-	// TODO: check that this covers all cases
-	if (exists.argument.is<ast::Comparison>())
+	return SimplificationRule::apply(formula);
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+template<class FirstSimplificationRule, class SecondSimplificationRule, class... OtherSimplificationRules>
+SimplificationResult simplify(ast::Formula &formula)
+{
+	if (simplify<FirstSimplificationRule>(formula) == SimplificationResult::Simplified)
+		return SimplificationResult::Simplified;
+
+	return simplify<SecondSimplificationRule, OtherSimplificationRules...>(formula);
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct SimplificationRuleExistsWithoutQuantifiedVariables
+{
+	static constexpr const auto Description = "exists () (F) === F";
+
+	static SimplificationResult apply(ast::Formula &formula)
 	{
+		if (!formula.is<ast::Exists>())
+			return SimplificationResult::Unchanged;
+
+		auto &exists = formula.get<ast::Exists>();
+
+		if (!exists.variables.empty())
+			return SimplificationResult::Unchanged;
+
+		formula = std::move(exists.argument);
+
+		return SimplificationResult::Simplified;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct SimplificationRuleTrivialAssignmentInExists
+{
+	static constexpr const auto Description = "exists X (X = Y) === #true";
+
+	static SimplificationResult apply(ast::Formula &formula)
+	{
+		if (!formula.is<ast::Exists>())
+			return SimplificationResult::Unchanged;
+
+		const auto &exists = formula.get<ast::Exists>();
+
+		if (!exists.argument.is<ast::Comparison>())
+			return SimplificationResult::Unchanged;
+
 		const auto &comparison = exists.argument.get<ast::Comparison>();
 
 		if (comparison.operator_ != ast::Comparison::Operator::Equal)
-			return;
+			return SimplificationResult::Unchanged;
 
 		const auto matchingAssignment = std::find_if(exists.variables.cbegin(), exists.variables.cend(),
 			[&](const auto &variableDeclaration)
@@ -117,107 +166,331 @@ void simplify(ast::Exists &exists, ast::Formula &formula)
 					|| matchesVariableDeclaration(comparison.right, *variableDeclaration);
 			});
 
-		if (matchingAssignment != exists.variables.cend())
-			formula = ast::Formula::make<ast::Boolean>(true);
+		if (matchingAssignment == exists.variables.cend())
+			return SimplificationResult::Unchanged;
 
-		return;
+		formula = ast::Formula::make<ast::Boolean>(true);
+
+		return SimplificationResult::Simplified;
 	}
+};
 
-	if (!exists.argument.is<ast::And>())
-		return;
+////////////////////////////////////////////////////////////////////////////////////////////////////
 
-	auto &conjunction = exists.argument.get<ast::And>();
-	auto &arguments = conjunction.arguments;
+struct SimplificationRuleAssignmentInExists
+{
+	static constexpr const auto Description = "exists X (X = t and F(X)) === exists () (F(t))";
 
-	// Simplify formulas of type “exists X (X = t and F(X))” to “F(t)”
-	for (auto i = exists.variables.begin(); i != exists.variables.end();)
+	static SimplificationResult apply(ast::Formula &formula)
 	{
-		const auto &variableDeclaration = **i;
+		if (!formula.is<ast::Exists>())
+			return SimplificationResult::Unchanged;
 
-		bool wasVariableReplaced = false;
+		auto &exists = formula.get<ast::Exists>();
 
-		// TODO: refactor
-		for (auto j = arguments.begin(); j != arguments.end(); j++)
+		if (!exists.argument.is<ast::And>())
+			return SimplificationResult::Unchanged;
+
+		auto &and_ = exists.argument.get<ast::And>();
+		auto &arguments = and_.arguments;
+
+		auto simplificationResult = SimplificationResult::Unchanged;
+
+		for (auto i = exists.variables.begin(); i != exists.variables.end();)
 		{
-			auto &argument = *j;
-			// Find term that is equivalent to the given variable
-			auto assignedTerm = extractAssignedTerm(argument, variableDeclaration);
+			const auto &variableDeclaration = **i;
 
-			if (!assignedTerm)
-				continue;
+			bool wasVariableReplaced = false;
 
-			// Replace all occurrences of the variable with the equivalent term
-			for (auto k = arguments.begin(); k != arguments.end(); k++)
+			// TODO: refactor
+			for (auto j = arguments.begin(); j != arguments.end(); j++)
 			{
-				if (k == j)
+				auto &argument = *j;
+				// Find term that is equivalent to the given variable
+				auto assignedTerm = extractAssignedTerm(argument, variableDeclaration);
+
+				if (!assignedTerm)
 					continue;
 
-				auto &otherArgument = *k;
-				otherArgument.accept(ReplaceVariableInFormulaVisitor(), otherArgument, variableDeclaration, assignedTerm.value());
+				// Replace all occurrences of the variable with the equivalent term
+				for (auto k = arguments.begin(); k != arguments.end(); k++)
+				{
+					if (k == j)
+						continue;
+
+					auto &otherArgument = *k;
+					otherArgument.accept(ReplaceVariableInFormulaVisitor(), otherArgument, variableDeclaration, assignedTerm.value());
+				}
+
+				arguments.erase(j);
+
+				wasVariableReplaced = true;
+				simplificationResult = SimplificationResult::Simplified;
+
+				break;
 			}
 
-			arguments.erase(j);
-			wasVariableReplaced = true;
-			break;
+			if (wasVariableReplaced)
+			{
+				i = exists.variables.erase(i);
+				continue;
+			}
+
+			i++;
 		}
 
-		if (wasVariableReplaced)
-		{
-			i = exists.variables.erase(i);
-			continue;
-		}
-
-		i++;
+		return simplificationResult;
 	}
+};
 
-	// If there are no arguments left, we had a formula of the form “exists X1, ..., Xn (X1 = Y1 and ... and Xn = Yn)”
-	// Such exists statements are useless and can be safely replaced with “#true”
-	if (arguments.empty())
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct SimplificationRuleEmptyConjunction
+{
+	static constexpr const auto Description = "[empty conjunction] === #true";
+
+	static SimplificationResult apply(ast::Formula &formula)
 	{
+		if (!formula.is<ast::And>())
+			return SimplificationResult::Unchanged;
+
+		auto &and_ = formula.get<ast::And>();
+
+		if (!and_.arguments.empty())
+			return SimplificationResult::Unchanged;
+
 		formula = ast::Formula::make<ast::Boolean>(true);
-		return;
+
+		return SimplificationResult::Simplified;
 	}
+};
 
-	// If the argument now is a conjunction with just one element, directly replace the input formula with the argument
-	if (arguments.size() == 1)
-		exists.argument = std::move(arguments.front());
+////////////////////////////////////////////////////////////////////////////////////////////////////
 
-	// If there are still remaining variables, simplification is over
-	if (!exists.variables.empty())
-		return;
+struct SimplificationRuleOneElementConjunction
+{
+	static constexpr const auto Description = "[conjunction of only F] === F";
 
-	assert(!arguments.empty());
+	static SimplificationResult apply(ast::Formula &formula)
+	{
+		if (!formula.is<ast::And>())
+			return SimplificationResult::Unchanged;
 
-	// If there is more than one element in the conjunction, replace the input formula with the conjunction
-	formula = std::move(exists.argument);
-}
+		auto &and_ = formula.get<ast::And>();
+
+		if (and_.arguments.size() != 1)
+			return SimplificationResult::Unchanged;
+
+		formula = std::move(and_.arguments.front());
+
+		return SimplificationResult::Simplified;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct SimplificationRuleTrivialExists
+{
+	static constexpr const auto Description = "exists ... ([#true/#false]) === [#true/#false]";
+
+	static SimplificationResult apply(ast::Formula &formula)
+	{
+		if (!formula.is<ast::Exists>())
+			return SimplificationResult::Unchanged;
+
+		auto &exists = formula.get<ast::Exists>();
+
+		if (!exists.argument.is<ast::Boolean>())
+			return SimplificationResult::Unchanged;
+
+		formula = std::move(exists.argument);
+
+		return SimplificationResult::Simplified;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct SimplificationRuleInWithPrimitiveArguments
+{
+	static constexpr const auto Description = "[primitive A] in [primitive B] === A = B";
+
+	static SimplificationResult apply(ast::Formula &formula)
+	{
+		if (!formula.is<ast::In>())
+			return SimplificationResult::Unchanged;
+
+		auto &in = formula.get<ast::In>();
+
+		assert(ast::isPrimitive(in.element));
+
+		if (!ast::isPrimitive(in.element) || !ast::isPrimitive(in.set))
+			return SimplificationResult::Unchanged;
+
+		formula = ast::Comparison(ast::Comparison::Operator::Equal, std::move(in.element), std::move(in.set));
+
+		return SimplificationResult::Simplified;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct SimplificationRuleSubsumptionInBiconditionals
+{
+	static constexpr const auto Description = "(F <-> (F and G)) === (F -> G)";
+
+	static SimplificationResult apply(ast::Formula &formula)
+	{
+		if (!formula.is<ast::Biconditional>())
+			return SimplificationResult::Unchanged;
+
+		auto &biconditional = formula.get<ast::Biconditional>();
+
+		const auto leftIsPredicate = biconditional.left.is<ast::Predicate>();
+		const auto rightIsPredicate = biconditional.right.is<ast::Predicate>();
+
+		const auto leftIsAnd = biconditional.left.is<ast::And>();
+		const auto rightIsAnd = biconditional.right.is<ast::And>();
+
+		if (!(leftIsPredicate && rightIsAnd) && !(rightIsPredicate && leftIsAnd))
+			return SimplificationResult::Unchanged;
+
+		auto &predicateSide = (leftIsPredicate ? biconditional.left : biconditional.right);
+		auto &andSide = (leftIsPredicate ? biconditional.right : biconditional.left);
+		auto &and_ = andSide.get<ast::And>();
+
+		const auto matchingPredicate =
+			std::find_if(and_.arguments.cbegin(), and_.arguments.cend(),
+			[&](const auto &argument)
+			{
+				return (ast::equal(predicateSide, argument) == ast::Tristate::True);
+			});
+
+		if (matchingPredicate == and_.arguments.cend())
+			return SimplificationResult::Unchanged;
+
+		and_.arguments.erase(matchingPredicate);
+
+		formula = ast::Formula::make<ast::Implies>(std::move(predicateSide), std::move(andSide));
+
+		return SimplificationResult::Simplified;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct SimplificationRuleDoubleNegation
+{
+	static constexpr const auto Description = "not not F === F";
+
+	static SimplificationResult apply(ast::Formula &formula)
+	{
+		if (!formula.is<ast::Not>())
+			return SimplificationResult::Unchanged;
+
+		auto &not_ = formula.get<ast::Not>();
+
+		if (!not_.argument.is<ast::Not>())
+			return SimplificationResult::Unchanged;
+
+		auto &notNot = not_.argument.get<ast::Not>();
+
+		formula = std::move(notNot.argument);
+
+		return SimplificationResult::Simplified;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct SimplificationRuleDeMorganForConjunctions
+{
+	static constexpr const auto Description = "(not (F and G)) === (not F or not G)";
+
+	static SimplificationResult apply(ast::Formula &formula)
+	{
+		if (!formula.is<ast::Not>())
+			return SimplificationResult::Unchanged;
+
+		auto &not_ = formula.get<ast::Not>();
+
+		if (!not_.argument.is<ast::And>())
+			return SimplificationResult::Unchanged;
+
+		auto &and_ = not_.argument.get<ast::And>();
+
+		for (auto &argument : and_.arguments)
+			argument = ast::Formula::make<ast::Not>(std::move(argument));
+
+		formula = ast::Formula::make<ast::Or>(std::move(and_.arguments));
+
+		return SimplificationResult::Simplified;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct SimplificationRuleImplicationFromDisjunction
+{
+	static constexpr const auto Description = "(not F or G) === (F -> G)";
+
+	static SimplificationResult apply(ast::Formula &formula)
+	{
+		if (!formula.is<ast::Or>())
+			return SimplificationResult::Unchanged;
+
+		auto &or_ = formula.get<ast::Or>();
+
+		if (or_.arguments.size() != 2)
+			return SimplificationResult::Unchanged;
+
+		const auto leftIsNot = or_.arguments[0].is<ast::Not>();
+		const auto rightIsNot = or_.arguments[1].is<ast::Not>();
+
+		if (leftIsNot == rightIsNot)
+			return SimplificationResult::Unchanged;
+
+		auto &negativeSide = leftIsNot ? or_.arguments[0] : or_.arguments[1];
+		auto &positiveSide = leftIsNot ? or_.arguments[1] : or_.arguments[0];
+
+		assert(negativeSide.is<ast::Not>());
+		assert(!positiveSide.is<ast::Not>());
+
+		auto &negativeSideArgument = negativeSide.get<ast::Not>().argument;
+
+		formula = ast::Formula::make<ast::Implies>(std::move(negativeSideArgument), std::move(positiveSide));
+
+		return SimplificationResult::Simplified;
+	}
+};
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+const auto simplifyWithDefaultRules =
+	simplify
+	<
+		SimplificationRuleDoubleNegation,
+		SimplificationRuleTrivialAssignmentInExists,
+		SimplificationRuleAssignmentInExists,
+		SimplificationRuleEmptyConjunction,
+		SimplificationRuleTrivialExists,
+		SimplificationRuleOneElementConjunction,
+		SimplificationRuleExistsWithoutQuantifiedVariables,
+		SimplificationRuleInWithPrimitiveArguments,
+		SimplificationRuleSubsumptionInBiconditionals,
+		SimplificationRuleDeMorganForConjunctions,
+		SimplificationRuleImplicationFromDisjunction
+	>;
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 // Performs the different simplification techniques
-struct SimplifyFormulaVisitor : public ast::RecursiveFormulaVisitor<SimplifyFormulaVisitor>
+struct SimplifyFormulaVisitor : public ast::FormulaSimplificationVisitor<SimplifyFormulaVisitor>
 {
-	// Forward exists statements to the dedicated simplification function
-	static void accept(ast::Exists &exists, ast::Formula &formula)
-	{
-		simplify(exists, formula);
-	}
-
-	// Simplify formulas of type “A in B” to “A = B” if A and B are primitive
-	static void accept(ast::In &in, ast::Formula &formula)
-	{
-		assert(ast::isPrimitive(in.element));
-
-		if (!ast::isPrimitive(in.element) || !ast::isPrimitive(in.set))
-			return;
-
-		formula = ast::Comparison(ast::Comparison::Operator::Equal, std::move(in.element), std::move(in.set));
-	}
-
 	// Do nothing for all other types of expressions
-	template<class T>
-	static void accept(T &, ast::Formula &)
+	static SimplificationResult accept(ast::Formula &formula)
 	{
+		return simplifyWithDefaultRules(formula);
 	}
 };
 
@@ -225,7 +498,7 @@ struct SimplifyFormulaVisitor : public ast::RecursiveFormulaVisitor<SimplifyForm
 
 void simplify(ast::Formula &formula)
 {
-	formula.accept(SimplifyFormulaVisitor(), formula);
+	while (formula.accept(SimplifyFormulaVisitor(), formula) == SimplificationResult::Simplified);
 }
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////

tests/TestCompletion.cpp

@@ -152,9 +152,9 @@ TEST_CASE("[completion] Rules are completed", "[completion]")
 
 		CHECK(output.str() ==
 			"forall V1 (covered(V1) <-> exists U1 in(V1, U1))\n"
-			"forall V2, V3 (in(V2, V3) <-> (V2 in 1..n and V3 in 1..r and in(V2, V3)))\n"
-			"forall U2 not (U2 in 1..n and not covered(U2))\n"
-			"forall U3, U4, U5 not (in(U3, U4) and in(U5, U4) and exists X1 (X1 in (U3 + U5) and in(X1, U4)))\n");
+			"forall V2, V3 (in(V2, V3) -> (V2 in 1..n and V3 in 1..r))\n"
+			"forall U2 (U2 in 1..n -> covered(U2))\n"
+			"forall U3, U4, U5 (not in(U3, U4) or not in(U5, U4) or not exists X1 (X1 in (U3 + U5) and in(X1, U4)))\n");
 	}
 
 	SECTION("binary operations with multiple variables")

tests/TestHiddenPredicateElimination.cpp

@@ -103,9 +103,10 @@ TEST_CASE("[hidden predicate elimination] Hidden predicates are correctly elimin
 			"#show a/1.";
 		anthem::translate("input", input, context);
 
+		// TODO: simplify further
 		CHECK(output.str() ==
 			"forall V1 (a(V1) <-> not d(V1))\n"
-			"forall V2 (d(V2) <-> not not d(V2))\n"
+			"forall V2 (d(V2) <-> d(V2))\n"
 			"forall V3 (e(V3) <-> e(V3))\n");
 	}
 
@@ -164,12 +165,11 @@ TEST_CASE("[hidden predicate elimination] Hidden predicates are correctly elimin
 			"#show t/0.";
 		anthem::translate("input", input, context);
 
-		// TODO: simplify further
 		CHECK(output.str() ==
-			"(s <-> (not #false and s))\n"
-			"(t <-> (not #false and t))\n"
-			"not (s and not t)\n"
-			"not (not #false and not #false and #false)\n");
+			"(s -> not #false)\n"
+			"(t -> not #false)\n"
+			"(s -> t)\n"
+			"(#false or #false or not #false)\n");
 	}
 
 	SECTION("predicate with more than one argument is hidden correctly")

tests/TestPlaceholders.cpp

@@ -55,8 +55,8 @@ TEST_CASE("[placeholders] Programs with placeholders are correctly completed", "
 		anthem::translate("input", input, context);
 
 		CHECK(output.str() ==
-			"forall V1, V2 (color(V1, V2) <-> (vertex(V1) and color(V2) and color(V1, V2)))\n"
-			"forall U1 not (vertex(U1) and not exists U2 color(U1, U2))\n"
-			"forall U3, U4, U5 not (color(U3, U4) and color(U5, U4) and edge(U3, U5))\n");
+			"forall V1, V2 (color(V1, V2) -> (vertex(V1) and color(V2)))\n"
+			"forall U1 (vertex(U1) -> exists U2 color(U1, U2))\n"
+			"forall U3, U4, U5 (not color(U3, U4) or not color(U5, U4) or not edge(U3, U5))\n");
 	}
 }