debugdatr.pl

% `Debug:<trace path...>` is meant to work as functionally
% a drop-in replacement for `path...`, except that it also
% prints it out to stderr.
% This can be useful if you want to avoid recomputing
% the local path (for example, if it's stateful and you want
% to avoid changing the meaning of the code)
'Debug'([trace|P], GN, GP, P) :- 'Debug'([writeline|P], GN, GP, []).

% `Debug:<writeline>` sends its output to stderr, so that you can
% avoid confusing it up with any actual output you may want to
% produce
'Debug'([writeline|P], GN, GP, []) :- !,
     with_output_to(user_error, 'System'([writeline|P], GN, GP, [])).