diff --git a/src/ast.rs b/src/ast.rs
index 7f764c3..0b6c92e 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -40,11 +40,70 @@ pub struct PredicateDeclaration
 	pub arity: usize,
 }
 
+#[derive(Eq, Hash, PartialEq)]
 pub struct VariableDeclaration
 {
 	pub name: String,
 }
 
+pub type VariableDeclarations = std::collections::HashSet<std::rc::Rc<VariableDeclaration>>;
+
+pub struct VariableDeclarationStack
+{
+	pub free_variable_declarations: VariableDeclarations,
+	bound_variable_declaration_stack: Vec<std::rc::Rc<VariableDeclarations>>,
+}
+
+impl VariableDeclarationStack
+{
+	pub fn find(&self, variable_name: &str) -> Option<std::rc::Rc<VariableDeclaration>>
+	{
+		for variable_declarations in self.bound_variable_declaration_stack.iter().rev()
+		{
+			if let Some(variable_declaration) = variable_declarations.iter().find(|x| x.name == variable_name)
+			{
+				return Some(std::rc::Rc::clone(&variable_declaration));
+			}
+		}
+
+		if let Some(variable_declaration) = self.free_variable_declarations.iter().find(|x| x.name == variable_name)
+		{
+			return Some(std::rc::Rc::clone(&variable_declaration));
+		}
+
+		None
+	}
+
+	pub fn find_or_create(&mut self, variable_name: &str) -> std::rc::Rc<VariableDeclaration>
+	{
+		if let Some(variable_declaration) = self.find(variable_name)
+		{
+			return variable_declaration;
+		}
+
+		let variable_declaration = VariableDeclaration
+		{
+			name: variable_name.to_owned(),
+		};
+		let variable_declaration = std::rc::Rc::new(variable_declaration);
+
+		self.free_variable_declarations.insert(std::rc::Rc::clone(&variable_declaration));
+
+		variable_declaration
+	}
+
+	pub fn push(&mut self, bound_variable_declarations: std::rc::Rc<VariableDeclarations>)
+	{
+		self.bound_variable_declaration_stack.push(bound_variable_declarations);
+	}
+
+	pub fn pop(&mut self)
+	{
+		// TODO: return error instead
+		self.bound_variable_declaration_stack.pop().expect("bound variable is empty, cannot pop last element");
+	}
+}
+
 // Terms
 
 pub struct BinaryOperation
diff --git a/src/parse.rs b/src/parse.rs
index 603141b..bcb0f64 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -10,53 +10,6 @@ use nom::
 	bytes::complete::tag,
 };
 
-type VariableDeclarations = std::collections::HashSet<std::rc::Rc<crate::VariableDeclaration>>;
-
-struct VariableDeclarationStack
-{
-	free_variable_declarations: VariableDeclarations,
-	bound_variable_declarations: Vec<std::rc::Rc<VariableDeclarations>>,
-}
-
-impl VariableDeclarationStack
-{
-	fn find(&self, variable_name: &str) -> Option<std::rc::Rc<crate::VariableDeclaration>>
-	{
-		for variable_declarations in self.iter().rev()
-		{
-			if let Some(variable_declaration) = variable_declarations.iter().find(|x| x.name == variable_name)
-			{
-				return Some(std::rc::Rc::clone(&variable_declaration));
-			}
-		}
-
-		if let Some(variable_declaration) = self.free_variable_declarations.iter().find(|x| x.name == variable_name)
-		{
-			return Some(std::rc::Rc::clone(&variable_declaration));
-		}
-
-		None
-	}
-
-	pub(crate) fn find_or_create(&mut self, variable_name: &str) -> std::rc::Rc<crate::VariableDeclaration>
-	{
-		if let Some(variable_declaration) = self.find(variable_name)
-		{
-			return variable_declaration;
-		}
-
-		let variable_declaration = crate::VariableDeclaration
-		{
-			name: variable_name.to_owned(),
-		};
-		let variable_declaration = std::rc::Rc::new(variable_declaration);
-
-		self.free_variable_declarations.insert(std::rc::Rc::clone(&variable_declaration));
-
-		variable_declaration
-	}
-}
-
 struct Declarations
 {
 	//function_declarations:,