use newly available must_be(chars, ...) for better and shorter type test
hex_bytes(Hs, Bytes) :-
( ground(Hs) ->
- must_be(list, Hs),
- maplist(must_be(atom), Hs),
+ must_be(chars, Hs),
( phrase(hex_bytes(Hs), Bytes) ->
true
; domain_error(hex_encoding, Hs, hex_bytes/2)
