diff --git a/lib/pddlparse/include/pddlparse/detail/parsing/Constant.h b/lib/pddlparse/include/pddlparse/detail/parsing/Constant.h
index eb227a3..172590d 100644
--- a/lib/pddlparse/include/pddlparse/detail/parsing/Constant.h
+++ b/lib/pddlparse/include/pddlparse/detail/parsing/Constant.h
@@ -16,6 +16,7 @@ namespace detail
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+std::experimental::optional<ast::ConstantPointer> testParsingConstant(Context &context, ASTContext &astContext);
 ast::ConstantPointer parseConstant(Context &context, ASTContext &astContext);
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
diff --git a/lib/pddlparse/include/pddlparse/detail/parsing/Predicate.h b/lib/pddlparse/include/pddlparse/detail/parsing/Predicate.h
index 760b7f6..d7fa173 100644
--- a/lib/pddlparse/include/pddlparse/detail/parsing/Predicate.h
+++ b/lib/pddlparse/include/pddlparse/detail/parsing/Predicate.h
@@ -17,6 +17,7 @@ namespace detail
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+// TODO: rename consistently
 std::experimental::optional<ast::PredicatePointer> parsePredicate(Context &context, ASTContext &astContext, VariableStack &variableStack);
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
diff --git a/lib/pddlparse/include/pddlparse/detail/parsing/Variable.h b/lib/pddlparse/include/pddlparse/detail/parsing/Variable.h
index b007d9e..bb8dc20 100644
--- a/lib/pddlparse/include/pddlparse/detail/parsing/Variable.h
+++ b/lib/pddlparse/include/pddlparse/detail/parsing/Variable.h
@@ -16,6 +16,8 @@ namespace detail
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+// TODO: find consistent naming scheme
+std::experimental::optional<ast::VariablePointer> testParsingVariable(Context &context, VariableStack &variableStack);
 ast::VariablePointer parseVariable(Context &context, VariableStack &variableStack);
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
diff --git a/lib/pddlparse/src/pddlparse/detail/parsing/Constant.cpp b/lib/pddlparse/src/pddlparse/detail/parsing/Constant.cpp
index 3cd8774..5fa3230 100644
--- a/lib/pddlparse/src/pddlparse/detail/parsing/Constant.cpp
+++ b/lib/pddlparse/src/pddlparse/detail/parsing/Constant.cpp
@@ -30,48 +30,8 @@ std::experimental::optional<ast::ConstantPointer> findConstant(const std::string
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-// TODO: remove if unneeded
-ast::ConstantPointer parseConstant(Context &context, ast::Domain &domain)
+std::experimental::optional<ast::ConstantPointer> findConstant(const std::string &constantName, ASTContext &astContext)
 {
-	auto &tokenizer = context.tokenizer;
-	const auto constantName = tokenizer.getIdentifier();
-
-	auto constant = findConstant(constantName, domain.constants);
-
-	if (constant)
-		return std::move(constant.value());
-
-	throw ParserException(tokenizer.location(), "constant “" + constantName + "” used but never declared");
-}
-
-////////////////////////////////////////////////////////////////////////////////////////////////////
-
-// TODO: remove if unneeded
-ast::ConstantPointer parseConstant(Context &context, ast::Problem &problem)
-{
-	auto &tokenizer = context.tokenizer;
-	const auto constantName = tokenizer.getIdentifier();
-
-	auto constant = findConstant(constantName, problem.domain->constants);
-
-	if (constant)
-		return std::move(constant.value());
-
-	auto object = findConstant(constantName, problem.objects);
-
-	if (object)
-		return std::move(object.value());
-
-	throw ParserException(tokenizer.location(), "constant “" + constantName + "” used but never declared");
-}
-
-////////////////////////////////////////////////////////////////////////////////////////////////////
-
-ast::ConstantPointer parseConstant(Context &context, ASTContext &astContext)
-{
-	auto &tokenizer = context.tokenizer;
-	const auto constantName = tokenizer.getIdentifier();
-
 	auto constant = findConstant(constantName, astContext.domain->constants);
 
 	if (constant)
@@ -85,7 +45,37 @@ ast::ConstantPointer parseConstant(Context &context, ASTContext &astContext)
 			return std::move(constant.value());
 	}
 
-	throw ParserException(tokenizer.location(), "constant “" + constantName + "” used but never declared");
+	return std::experimental::nullopt;
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+std::experimental::optional<ast::ConstantPointer> testParsingConstant(Context &context, ASTContext &astContext)
+{
+	auto &tokenizer = context.tokenizer;
+
+	const auto constantName = tokenizer.getIdentifier();
+	auto constant = findConstant(constantName, astContext);
+
+	if (!constant)
+		return std::experimental::nullopt;
+
+	return std::move(constant.value());
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+ast::ConstantPointer parseConstant(Context &context, ASTContext &astContext)
+{
+	auto &tokenizer = context.tokenizer;
+
+	const auto constantName = tokenizer.getIdentifier();
+	auto constant = findConstant(constantName, astContext);
+
+	if (!constant)
+		throw ParserException(tokenizer.location(), "undeclared constant “" + constantName + "”");
+
+	return std::move(constant.value());
 }
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
diff --git a/lib/pddlparse/src/pddlparse/detail/parsing/Predicate.cpp b/lib/pddlparse/src/pddlparse/detail/parsing/Predicate.cpp
index b8a2fcb..b3aecbc 100644
--- a/lib/pddlparse/src/pddlparse/detail/parsing/Predicate.cpp
+++ b/lib/pddlparse/src/pddlparse/detail/parsing/Predicate.cpp
@@ -20,11 +20,11 @@ std::experimental::optional<ast::PredicatePointer> parsePredicate(Context &conte
 {
 	auto &tokenizer = context.tokenizer;
 
-	const auto position = tokenizer.position();
+	const auto previousPosition = tokenizer.position();
 
 	if (!tokenizer.testAndSkip<std::string>("("))
 	{
-		tokenizer.seek(position);
+		tokenizer.seek(previousPosition);
 		return std::experimental::nullopt;
 	}
 
@@ -53,14 +53,31 @@ std::experimental::optional<ast::PredicatePointer> parsePredicate(Context &conte
 	// Parse arguments
 	while (tokenizer.currentCharacter() != ')')
 	{
-		// Parse variables
-		if (tokenizer.currentCharacter() == '?')
-			arguments.emplace_back(parseVariable(context, variableStack));
-		// Parse constants
-		else
-			arguments.emplace_back(parseConstant(context, astContext));
+		// Parse argument if it is a variable
+		auto variable = testParsingVariable(context, variableStack);
 
-		tokenizer.skipWhiteSpace();
+		if (variable)
+		{
+			arguments.emplace_back(std::move(variable.value()));
+
+			tokenizer.skipWhiteSpace();
+			continue;
+		}
+
+		// Parse argument if it is a constant
+		auto constant = testParsingConstant(context, astContext);
+
+		if (constant)
+		{
+			arguments.emplace_back(std::move(constant.value()));
+
+			tokenizer.skipWhiteSpace();
+			continue;
+		}
+
+		// If argument is neither variable nor constant, this is not a valid predicate
+		tokenizer.seek(previousPosition);
+		return std::experimental::nullopt;
 	}
 
 	//const auto &predicates = astContext.domain->predicates;
diff --git a/lib/pddlparse/src/pddlparse/detail/parsing/Variable.cpp b/lib/pddlparse/src/pddlparse/detail/parsing/Variable.cpp
index 31249a3..1da68be 100644
--- a/lib/pddlparse/src/pddlparse/detail/parsing/Variable.cpp
+++ b/lib/pddlparse/src/pddlparse/detail/parsing/Variable.cpp
@@ -14,14 +14,53 @@ namespace detail
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-ast::VariablePointer parseVariable(Context &context, VariableStack &variableStack)
+std::experimental::optional<std::string> testParsingVariableName(Context &context)
+{
+	auto &tokenizer = context.tokenizer;
+
+	if (!tokenizer.testAndReturn<std::string>("?"))
+		return std::experimental::nullopt;
+
+	tokenizer.expect<std::string>("?");
+
+	return tokenizer.getIdentifier();
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+std::string parseVariableName(Context &context)
 {
 	auto &tokenizer = context.tokenizer;
 
 	tokenizer.expect<std::string>("?");
 
-	const auto variableName = tokenizer.getIdentifier();
+	return tokenizer.getIdentifier();
+}
 
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+std::experimental::optional<ast::VariablePointer> testParsingVariable(Context &context, VariableStack &variableStack)
+{
+	auto variableName = testParsingVariableName(context);
+
+	if (!variableName)
+		return std::experimental::nullopt;
+
+	auto variableDeclaration = variableStack.findVariableDeclaration(variableName.value());
+
+	if (!variableDeclaration)
+		return std::experimental::nullopt;
+
+	return std::make_unique<ast::Variable>(variableDeclaration.value());
+}
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+ast::VariablePointer parseVariable(Context &context, VariableStack &variableStack)
+{
+	auto &tokenizer = context.tokenizer;
+
+	auto variableName = parseVariableName(context);
 	auto variableDeclaration = variableStack.findVariableDeclaration(variableName);
 
 	if (!variableDeclaration)