diff --git a/src/negation_normal_form/tests.rs b/src/negation_normal_form/tests.rs
index 8318195..3eafadc 100644
--- a/src/negation_normal_form/tests.rs
+++ b/src/negation_normal_form/tests.rs
@@ -13,9 +13,12 @@ mod tests {
 
     #[test]
     fn double_neg() {
-        assert_eq!(negation_normal_form("A!!B&"), "AB&");
-        assert_eq!(negation_normal_form("A!!B!!&"), "AB&");
-        assert_eq!(negation_normal_form("A!!B!&"), "AB!&");
+        assert_eq!(negation_normal_form("A!!A&"), "AA&");
+        assert_eq!(negation_normal_form("B!!B!!&"), "BB&");
+        assert_eq!(negation_normal_form("C!!C!&"), "CC!&");
+        assert_eq!(negation_normal_form("D!!D&!!!!"), "DD&");
+        assert_eq!(negation_normal_form("E!!E!!!!&!!"), "EE&");
+        assert_eq!(negation_normal_form("F!!F!&!!!!!!!!"), "FF!&");
     }
 
     #[test]