%!demo
%! ## For a categorical array `ismissing` is the same test as `isundefined`: the
%! ## missing value *is* the `<undefined>` element.  It exists so categoricals
%! ## plug into the generic missing-data tools (`rmmissing`, `standardizeMissing`).
%!
%! C = categorical ({'M'; ''; 'L'})
%! ismissing (C)
