diff --git a/src/ast.rs b/src/ast.rs
index 06ebb77..08786ce 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -53,7 +53,7 @@ pub enum Formula
 	Predicate(Predicate),
 }
 
-#[derive(PartialEq)]
+#[derive(Eq, Hash, PartialEq)]
 pub enum Domain
 {
 	Program,