diff --git a/include/anthem/AST.h b/include/anthem/AST.h
index 87c20d1..bc97e47 100644
--- a/include/anthem/AST.h
+++ b/include/anthem/AST.h
@@ -243,9 +243,14 @@ struct PredicateDeclaration
 		Hidden
 	};
 
+	struct Parameter
+	{
+		Domain domain{Domain::Unknown};
+	};
+
 	explicit PredicateDeclaration(std::string &&name, size_t arity)
 	:	name{std::move(name)},
-		arity{arity}
+		parameters{std::vector<Parameter>(arity)}
 	{
 	}
 
@@ -254,8 +259,13 @@ struct PredicateDeclaration
 	PredicateDeclaration(PredicateDeclaration &&other) noexcept = default;
 	PredicateDeclaration &operator=(PredicateDeclaration &&other) noexcept = default;
 
+	size_t arity() const noexcept
+	{
+		return parameters.size();
+	}
+
 	std::string name;
-	size_t arity;
+	std::vector<Parameter> parameters;
 	bool isUsed{false};
 	bool isExternal{false};
 	Visibility visibility{Visibility::Default};
diff --git a/include/anthem/Context.h b/include/anthem/Context.h
index 06cadda..a341a43 100644
--- a/include/anthem/Context.h
+++ b/include/anthem/Context.h
@@ -38,7 +38,7 @@ struct Context
 		const auto matchesExistingPredicateDeclaration =
 			[&](const auto &predicateDeclaration)
 			{
-				return (predicateDeclaration->arity == arity
+				return (predicateDeclaration->arity() == arity
 					&& strcmp(predicateDeclaration->name.c_str(), name) == 0);
 			};
 
diff --git a/include/anthem/output/AST.h b/include/anthem/output/AST.h
index b12cee4..7aa1957 100644
--- a/include/anthem/output/AST.h
+++ b/include/anthem/output/AST.h
@@ -261,7 +261,7 @@ inline output::ColorStream &print(output::ColorStream &stream, const Predicate &
 
 inline output::ColorStream &print(output::ColorStream &stream, const PredicateDeclaration &predicateDeclaration, PrintContext &, bool)
 {
-	return (stream << predicateDeclaration.name << "/" << predicateDeclaration.arity);
+	return (stream << predicateDeclaration.name << "/" << predicateDeclaration.arity());
 }
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
diff --git a/src/anthem/Completion.cpp b/src/anthem/Completion.cpp
index a4e76fc..c1f1507 100644
--- a/src/anthem/Completion.cpp
+++ b/src/anthem/Completion.cpp
@@ -104,12 +104,12 @@ ast::Formula completePredicate(ast::PredicateDeclaration &predicateDeclaration,
 {
 	// Create new set of parameters for the completed definition for the predicate
 	ast::VariableDeclarationPointers parameters;
-	parameters.reserve(predicateDeclaration.arity);
+	parameters.reserve(predicateDeclaration.arity());
 
 	std::vector<ast::Term> arguments;
-	arguments.reserve(predicateDeclaration.arity);
+	arguments.reserve(predicateDeclaration.arity());
 
-	for (size_t i = 0; i < predicateDeclaration.arity; i++)
+	for (size_t i = 0; i < predicateDeclaration.arity(); i++)
 	{
 		parameters.emplace_back(std::make_unique<ast::VariableDeclaration>(ast::VariableDeclaration::Type::Head));
 		arguments.emplace_back(ast::Term::make<ast::Variable>(parameters.back().get()));
@@ -169,7 +169,7 @@ std::vector<ast::Formula> complete(std::vector<ast::ScopedFormula> &&scopedFormu
 			if (order != 0)
 				return (order < 0);
 
-			return lhs->arity < rhs->arity;
+			return lhs->arity() < rhs->arity();
 		});
 
 	std::vector<ast::Formula> completedFormulas;
diff --git a/src/anthem/HiddenPredicateElimination.cpp b/src/anthem/HiddenPredicateElimination.cpp
index 85ec768..375248a 100644
--- a/src/anthem/HiddenPredicateElimination.cpp
+++ b/src/anthem/HiddenPredicateElimination.cpp
@@ -181,7 +181,7 @@ PredicateReplacement findReplacement(const ast::PredicateDeclaration &predicateD
 	else if (completedPredicateDefinition.is<ast::Not>())
 		return findReplacement(predicateDeclaration, completedPredicateDefinition.get<ast::Not>());
 
-	throw CompletionException("unsupported completed definition for predicate “" + predicateDeclaration.name + "/" +  std::to_string(predicateDeclaration.arity) + "” for hiding predicates");
+	throw CompletionException("unsupported completed definition for predicate “" + predicateDeclaration.name + "/" +  std::to_string(predicateDeclaration.arity()) + "” for hiding predicates");
 }
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
@@ -217,7 +217,7 @@ void eliminateHiddenPredicates(std::vector<ast::Formula> &completedFormulas, Con
 		if (isPredicateVisible)
 			continue;
 
-		context.logger.log(output::Priority::Debug) << "eliminating “" << predicateDeclaration->name << "/" << predicateDeclaration->arity << "”";
+		context.logger.log(output::Priority::Debug) << "eliminating “" << predicateDeclaration->name << "/" << predicateDeclaration->arity() << "”";
 
 		const auto &completedPredicateDefinition = completedFormulas[i];
 		auto replacement = findReplacement(*predicateDeclaration, completedPredicateDefinition);
@@ -227,7 +227,7 @@ void eliminateHiddenPredicates(std::vector<ast::Formula> &completedFormulas, Con
 
 		if (hasCircularDependency)
 		{
-			context.logger.log(output::Priority::Warning) << "cannot hide predicate “" << predicateDeclaration->name << "/" << predicateDeclaration->arity << "” due to circular dependency";
+			context.logger.log(output::Priority::Warning) << "cannot hide predicate “" << predicateDeclaration->name << "/" << predicateDeclaration->arity() << "” due to circular dependency";
 			continue;
 		}
 
diff --git a/src/anthem/Translation.cpp b/src/anthem/Translation.cpp
index 4772d97..58bfa06 100644
--- a/src/anthem/Translation.cpp
+++ b/src/anthem/Translation.cpp
@@ -96,7 +96,7 @@ void translate(const char *fileName, std::istream &stream, Context &context)
 				<< "#show declaration of “"
 				<< predicateDeclaration->name
 				<< "/"
-				<< predicateDeclaration->arity
+				<< predicateDeclaration->arity()
 				<< "” does not match any declared predicate";
 
 		// Check for #external statements with undeclared predicates
@@ -105,7 +105,7 @@ void translate(const char *fileName, std::istream &stream, Context &context)
 				<< "#external declaration of “"
 				<< predicateDeclaration->name
 				<< "/"
-				<< predicateDeclaration->arity
+				<< predicateDeclaration->arity()
 				<< "” does not match any declared predicate";
 	}