Commits on Apr 11, 2021

1. Check arguments to FreeSemigroup more thoroughly …

   See gap-system#1385
   
   It was previously possible to use `FreeSemigroup` to create a
   seemingly free semigroup with zero generators (rank zero), but
   this object believed itself to be infinite, and to contain elements
   (such a semigroup would be empty). Note that free semigroups of rank
   zero are not documented as being supported.
   
   Some other ways that one might want to create free semigroups of
   rank zero led to unhelpful error messages, and in general, the
   argument checking of `FreeSemigroup` was not very thorough and gave
   sometimes unhelpful error messages.
   
   In this commit, I overhaul the way that `FreeSemigroup` checks its
   arguments, in particular always disallowing a free semigroup of rank
   zero, and overall making the checking more robust, and making the error
   messages more descriptive.
   
   This addresses the _bugs_ reported in issue gap-system#1385, although it does not
   address the _feature request_ in that issue to support free semigroups of
   rank zero.
   
   I also took the opportunity to fix a typo in the documentation
   for `FreeSemigroup`, and to make a clarification. Further improvements
   could be made to this documentation, but I leave that to the future.

   

   wilfwilson committed Apr 11, 2021
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