%!demo
%! ## `isundefined` flags the elements that belong to no category — the
%! ## `<undefined>` values that stand in for missing data.  Empty labels become
%! ## undefined at construction time.
%!
%! C = categorical ({'M'; ''; 'L'; 'M'})
%! isundefined (C)
%!
%! ## Dropping a category also undefines every element that used it.
%! isundefined (removecats (C, 'M'))
