Assert against empty parameter lists

src/ast.rs

@@ -502,6 +502,8 @@ impl Formula
 
 	pub fn exists(parameters: std::rc::Rc<VariableDeclarations>, argument: Box<Formula>) -> Self
 	{
+		assert!(!parameters.is_empty());
+
 		Self::Exists(Exists::new(parameters, argument))
 	}
 
@@ -517,6 +519,8 @@ impl Formula
 
 	pub fn for_all(parameters: std::rc::Rc<VariableDeclarations>, argument: Box<Formula>) -> Self
 	{
+		assert!(!parameters.is_empty());
+
 		Self::ForAll(ForAll::new(parameters, argument))
 	}