diff --git a/src/ast.rs b/src/ast.rs
index 3cd9326..3cb94e0 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -47,7 +47,7 @@ impl FunctionDeclaration
 	}
 }
 
-pub type FunctionDeclarations = std::collections::HashSet<std::rc::Rc<FunctionDeclaration>>;
+pub type FunctionDeclarations = std::collections::BTreeSet<std::rc::Rc<FunctionDeclaration>>;
 
 #[derive(Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct PredicateDeclaration
@@ -68,7 +68,7 @@ impl PredicateDeclaration
 	}
 }
 
-pub type PredicateDeclarations = std::collections::HashSet<std::rc::Rc<PredicateDeclaration>>;
+pub type PredicateDeclarations = std::collections::BTreeSet<std::rc::Rc<PredicateDeclaration>>;
 
 #[derive(Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub struct VariableDeclaration
diff --git a/src/format.rs b/src/format.rs
index 1aca79e..b88b8af 100644
--- a/src/format.rs
+++ b/src/format.rs
@@ -211,7 +211,7 @@ impl<'formula> std::fmt::Debug for FormulaDisplay<'formula>
 
 				let mut separator = " ";
 
-				for parameter in &exists.parameters
+				for parameter in exists.parameters.iter()
 				{
 					write!(format, "{}{:?}", separator, parameter)?;
 
@@ -226,7 +226,7 @@ impl<'formula> std::fmt::Debug for FormulaDisplay<'formula>
 
 				let mut separator = " ";
 
-				for parameter in &for_all.parameters
+				for parameter in for_all.parameters.iter()
 				{
 					write!(format, "{}{:?}", separator, parameter)?;