whitespace? calls between? with result-type being nil
This leads to an error in between? because it tries to call (coerce nil nil), which (at least on SBCL) results in an error.
Note that NIL is not really a type. Did you mean NULL (the type of NIL)?
(coerce nil nil)
Replace coerce with map to guarantee consistency in between?. Fixes #5.
Thank for the report. I didn't notice that map handles nil specially, but coerce does not. Changed to mapping over nil, which look a bit strange but at least is always consistent regarding the expected result type.
Thank you for your quick reaction :)