diff --git a/src/format/formulas.rs b/src/format/formulas.rs
index 47d25f0..cfeaf74 100644
--- a/src/format/formulas.rs
+++ b/src/format/formulas.rs
@@ -378,16 +378,16 @@ mod tests
 	{
 		($intermediate:ident, $formula:expr, $output:expr) =>
 		{
-			let $intermediate = |f| f;
+			let $intermediate = |f: Box<Formula>| f;
 			assert!($formula, $output);
 
-			let $intermediate = |f| and(vec![f]);
+			let $intermediate = |f: Box<Formula>| and(vec![f]);
 			assert!($formula, $output);
 
-			let $intermediate = |f| or(vec![f]);
+			let $intermediate = |f: Box<Formula>| or(vec![f]);
 			assert!($formula, $output);
 
-			let $intermediate = |f| if_and_only_if(vec![f]);
+			let $intermediate = |f: Box<Formula>| if_and_only_if(vec![f]);
 			assert!($formula, $output);
 		};
 	}