diff --git a/src/ast.rs b/src/ast.rs
index 041a7aa..d270106 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -52,7 +52,7 @@ pub struct VariableDeclaration
 	pub name: String,
 }
 
-pub type VariableDeclarations = std::collections::HashSet<std::rc::Rc<VariableDeclaration>>;
+pub type VariableDeclarations = Vec<std::rc::Rc<VariableDeclaration>>;
 
 pub struct VariableDeclarationStack
 {
@@ -102,7 +102,7 @@ impl VariableDeclarationStack
 		};
 		let variable_declaration = std::rc::Rc::new(variable_declaration);
 
-		self.free_variable_declarations.insert(std::rc::Rc::clone(&variable_declaration));
+		self.free_variable_declarations.push(std::rc::Rc::clone(&variable_declaration));
 
 		variable_declaration
 	}
@@ -162,13 +162,13 @@ pub struct Compare
 
 pub struct Exists
 {
-	pub parameters: Vec<std::rc::Rc<VariableDeclaration>>,
+	pub parameters: VariableDeclarations,
 	pub argument: Box<Formula>,
 }
 
 pub struct ForAll
 {
-	pub parameters: Vec<std::rc::Rc<VariableDeclaration>>,
+	pub parameters: VariableDeclarations,
 	pub argument: Box<Formula>,
 }