phone-number: no-op declare compliance with 1.6.1
phone-number: no-op declare compliance with 1.6.1

1.6.0 uses error objects to distinguish between different kinds of error
(currently represented as `None` in Haskell)
https://github.com/exercism/problem-specifications/pull/1337

Potentially we could change to using `Either` and distinguishing between
the various types of errors, but I don't really see this as making the
exercise any more interesting.

1.6.1: adds a comment, no effect on tests
https://github.com/exercism/problem-specifications/pull/1361

exercism/xhaskell