diff --git a/src/ast.rs b/src/ast.rs
index 601ab18..7fa5ccb 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -28,7 +28,7 @@ pub enum UnaryOperator
 
 // Primitives
 
-#[derive(Eq, Hash, PartialEq)]
+#[derive(Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct FunctionDeclaration
 {
 	pub name: String,
@@ -37,7 +37,7 @@ pub struct FunctionDeclaration
 
 pub type FunctionDeclarations = std::collections::HashSet<std::rc::Rc<FunctionDeclaration>>;
 
-#[derive(Eq, Hash, PartialEq)]
+#[derive(Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct PredicateDeclaration
 {
 	pub name: String,
@@ -46,7 +46,7 @@ pub struct PredicateDeclaration
 
 pub type PredicateDeclarations = std::collections::HashSet<std::rc::Rc<PredicateDeclaration>>;
 
-#[derive(Eq, Hash, PartialEq)]
+#[derive(Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct VariableDeclaration
 {
 	pub name: String,