val two: level
-val unknown: status -> Marks.mark -> level
+val unknown: status -> level
-val is_zero: level -> bool
+val minus: status -> level -> int -> level
-val minus: level -> int -> level
+val to_string: status -> level -> string
-val to_string: level -> string
+val assert_not_zero: status -> level -> bool
+
+val assert_zero: status -> level -> bool
+
+val assert_equal: status -> level -> level -> bool