diff --git a/src/ast.rs b/src/ast.rs
index dac7740..5c5093b 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -35,6 +35,8 @@ pub struct FunctionDeclaration
 	pub arity: usize,
 }
 
+pub type FunctionDeclarations = std::collections::HashSet<std::rc::Rc<FunctionDeclaration>>;
+
 #[derive(Eq, Hash, PartialEq)]
 pub struct PredicateDeclaration
 {
@@ -42,6 +44,8 @@ pub struct PredicateDeclaration
 	pub arity: usize,
 }
 
+pub type PredicateDeclarations = std::collections::HashSet<std::rc::Rc<PredicateDeclaration>>;
+
 #[derive(Eq, Hash, PartialEq)]
 pub struct VariableDeclaration
 {