%!demo
%! ## `categories` lists an array's categories — the full declared label set, not
%! ## merely the values that happen to be present.
%!
%! C = categorical ({'M'; 'S'; 'M'}, {'S', 'M', 'L'}, 'Ordinal', true)
%! categories (C)
%!
%! ## `L` is a declared category (and, being ordinal, keeps its place in the
%! ## order) even though no element currently takes that value.
