diff --git a/include/anthem/AST.h b/include/anthem/AST.h
index 79c3505..9157fb9 100644
--- a/include/anthem/AST.h
+++ b/include/anthem/AST.h
@@ -14,6 +14,12 @@ namespace ast
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+// Terms are primitive (or arguments) if they are neither operations nor intervals
+inline bool isPrimitive(const ast::Term &term)
+{
+	return (!term.is<ast::BinaryOperation>() && !term.is<ast::Interval>());
+}
+
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 // Primitives
 ////////////////////////////////////////////////////////////////////////////////////////////////////
@@ -112,12 +118,15 @@ struct Function
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+// TODO: refactor (limit element type to primitive terms)
 struct In
 {
 	In(Term &&element, Term &&set)
 	:	element{std::move(element)},
 		set{std::move(set)}
 	{
+		// While the set may be any term, the element must be primitive
+		assert(isPrimitive(element));
 	}
 
 	Term element;
diff --git a/src/anthem/Simplification.cpp b/src/anthem/Simplification.cpp
index aa1b406..5340c2b 100644
--- a/src/anthem/Simplification.cpp
+++ b/src/anthem/Simplification.cpp
@@ -11,16 +11,6 @@ namespace anthem
 //
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
-// Determins whether a term is primitive
-// All terms but binary operations and intervals are primitive
-// With primitive terms t, “X in t” and “X = t” are equivalent
-bool isPrimitiveTerm(const ast::Term &term)
-{
-	return (!term.is<ast::BinaryOperation>() && !term.is<ast::Interval>());
-}
-
-////////////////////////////////////////////////////////////////////////////////////////////////////
-
 // Determines whether a term is a specific variable
 bool matchesVariable(const ast::Term &term, const ast::Variable &variable)
 {
@@ -189,7 +179,9 @@ struct SimplifyFormulaVisitor : public ast::RecursiveFormulaVisitor<SimplifyForm
 	// 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)
 	{
-		if (!isPrimitiveTerm(in.element) || !isPrimitiveTerm(in.set))
+		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));