our_ansi_format/3 errors out when allowed to backtrack.

[stassa]

our_ansi_format/3 is a thin wrapper around SWI-Prolog's ansi_format/3 to allow passing colour arguments as atoms rather than lists of attributes. The following example is from the SWI-Prolog documentation for ansi_format/3.

% Start in the Prolog shell but load metta_interp.pl.
stassaP@VARYS:~$ cd /mnt/c/Users/me/Documents/Prolog/metta/metta-wam_win
stassaP@VARYS:/mnt/c/Users/meDocuments/Prolog/metta/metta-wam_win$ swipl $PWD/prolog/metta_lang/metta_interp.pl --prolog

% Note the ; after the first successful call.
1 ?- our_ansi_format([bold,fg(cyan)], 'Hello ~w', [world]).
Hello world
true ;
% Term-position in <clause>(0x561914d278c0) at PC=27:
% eof_hook(register,file_scope,'/home/me/lib/swipl/library/prolog_clause.pl',set_prolog_flag(dialect_pfc,default))
% eof_hook(register,file_scope,'/home/me/lib/swipl/library/prolog_clause.pl',set_prolog_flag(subclause_expansion,true))
% info(load_mpred_file_complete(prolog_clause:'/home/me/lib/swipl/library/prolog_clause.pl'))
% info(load_mpred_file_complete(prolog_clause:'/home/me/lib/swipl/library/prolog_clause.pl'))
% found_eof_hook((t_l:eof_hook('/home/me/lib/swipl/library/prolog_clause.pl',file_scope:set_prolog_flag(subclause_expansion,true)):-true))
% call_eof_hook(on_f_log_ignore(prolog_clause,t_l:eof_hook('/home/me/lib/swipl/library/prolog_clause.pl',file_scope:set_prolog_flag(subclause_expansion,true))))
% found_eof_hook((t_l:eof_hook('/home/me/lib/swipl/library/prolog_clause.pl',file_scope:set_prolog_flag(dialect_pfc,default)):-true))
% call_eof_hook(on_f_log_ignore(prolog_clause,t_l:eof_hook('/home/me/lib/swipl/library/prolog_clause.pl',file_scope:set_prolog_flag(dialect_pfc,default))))
% clause_info(<clause>(0x561914d278c0)) (1-st clause of rgb/4)...
% from /home/me/lib/swipl/library/ansi_term.pl:319 ...
% eof_hook(register,file_scope,'/home/me/lib/swipl/library/prolog_source.pl',set_prolog_flag(dialect_pfc,default))
% eof_hook(register,file_scope,'/home/me/lib/swipl/library/prolog_source.pl',set_prolog_flag(subclause_expansion,true))
% Trying with syntax prolog
% eof_hook(register,file_scope,'/home/me/lib/swipl/library/operators.pl',set_prolog_flag(dialect_pfc,default))
% eof_hook(register,file_scope,'/home/me/lib/swipl/library/operators.pl',set_prolog_flag(subclause_expansion,true))
% read ...
% unified ...
% got names

%       <span class="pl-list"><span class="pl-functor"> [ </span><span class="pl-args"><span class="pl-int">2</span>, <span class="pl-int">1</span></span>]</span>

% eof_hook(register,file_scope,'/home/me/lib/swipl/library/prolog_clause.pl',set_prolog_flag(dialect_pfc,default))
% eof_hook(register,file_scope,'/home/me/lib/swipl/library/prolog_clause.pl',set_prolog_flag(subclause_expansion,true))
% Term-position in <clause>(0x561914d26fc0) at PC=44:
% clause_info(<clause>(0x561914d26fc0)) (16-th clause of sgr_code/2)...
% from /home/me/lib/swipl/library/ansi_term.pl:244 ...
% Trying with syntax prolog
% read ...
% unified ...
% got names

%       <span class="pl-list"><span class="pl-functor"> [ </span><span class="pl-args"><span class="pl-int">2</span>, <span class="pl-int">2</span>|<span class="pl-ellipsis">...</span></span>]</span>

% Term-position in <clause>(0x561914ce5b80) at PC=9:
% clause_info(<clause>(0x561914ce5b80)) (4-th clause of sgr_codes_ex/3)...
% from /home/me/lib/swipl/library/ansi_term.pl:176 ...
% Trying with syntax prolog
% read ...
% eof_hook(register,file_scope,'/home/me/lib/swipl/library/occurs.pl',set_prolog_flag(dialect_pfc,default))
% eof_hook(register,file_scope,'/home/me/lib/swipl/library/occurs.pl',set_prolog_flag(subclause_expansion,true))
% unified ...
% got names

%       <span class="pl-list"><span class="pl-functor"> [ </span><span class="pl-args"><span class="pl-int">2</span>, <span class="pl-int">1</span>|<span class="pl-ellipsis">...</span></span>]</span>

% Term-position in <clause>(0x561914d13080) at PC=16:
% clause_info(<clause>(0x561914d13080)) (3-th clause of sgr_codes_ex/3)...
% from /home/me/lib/swipl/library/ansi_term.pl:172 ...
% Trying with syntax prolog
% read ...
% unified ...
% got names

%       <span class="pl-list"><span class="pl-functor"> [ </span><span class="pl-args"><span class="pl-int">2</span>, <span class="pl-int">2</span>|<span class="pl-ellipsis">...</span></span>]</span>

% Term-position in <clause>(0x561914beec80) at PC=60:
% clause_info(<clause>(0x561914beec80)) (1-st clause of call_dcg/3)...
% from /home/me/lib/swipl/boot/dcg.pl:352 ...
% Trying with syntax prolog
% read ...
% unified ...
% got names

%       <span class="pl-list"><span class="pl-functor"> [ </span><span class="pl-args"><span class="pl-int">2</span>, <span class="pl-int">2</span></span>]</span>

% Term-position in <clause>(0x561914cd7b00) at PC=30:
% clause_info(<clause>(0x561914cd7b00)) (1-st clause of ansi_format/4)...
% from /home/me/lib/swipl/library/ansi_term.pl:147 ...
% Trying with syntax prolog
% read ...
% unified ...
% got names

%       <span class="pl-functor">[ </span>
  <span class="pl-int">2</span>, <span class="pl-int">2</span>, <span class="pl-int">2</span>, <span class="pl-int">2</span>, <span class="pl-int">2</span>, <span class="pl-int">1</span>]

% Term-position in <clause>(0x5619168fd0a0) at PC=18:
% clause_info(<clause>(0x5619168fd0a0)) (1-st clause of our_ansi_format/3)...
% from /mnt/c/Users/YeGoblynQueenne/Documents/Prolog/metta/metta-wam_win/prolog/metta_lang/metta_testing.pl:249 ...
% Trying with syntax prolog
% read ...
% unified ...
% got names

%       <span class="pl-list"><span class="pl-functor"> [ </span><span class="pl-args"><span class="pl-int">2</span>, <span class="pl-int">2</span></span>]</span>

ERROR: Type error: `atom' expected, found `
[ bold, fg(cyan)]

                                           ' (a list)
ERROR: In:
ERROR:   [53] atom_codes(
[ bold, fg(cyan)]

                         ,
[ 35, _38212, _38218, _38224, _38230, _38236, _3270]

                          )
ERROR:   [52] ansi_term:rgb(
[ bold, fg(cyan)]

                            ,_834,_836,_838) at /home/me/lib/swipl/library/ansi_term.pl:320
ERROR:   [51] ansi_term:sgr_code(fg(
[ bold, fg(cyan)]

                                    ),_886) at /home/me/lib/swipl/library/ansi_term.pl:247
ERROR:   [50] ansi_term:sgr_codes_ex(fg(
[ bold, fg(cyan)]

                                        ),_938,_940) at /home/me/lib/swipl/library/ansi_term.pl:177
ERROR:   [49] ansi_term:sgr_codes_ex(
[ fg( [ bold

                                     ,_992,[]) at /home/me/lib/swipl/library/ansi_term.pl:174
ERROR:   [48] '$dcg':call_dcg(ansi_term:sgr_codes_ex(
[ fg( [ bold

                                                     ),_1052,[]) at /home/me/lib/swipl/boot/dcg.pl:359
ERROR:   [45] ansi_term:ansi_format(current_output,
[ fg( [ bold

                                                   ,'Hello ~w',
[ world]

                                                               ) at /home/me/lib/swipl/library/ansi_term.pl:152
ERROR:   [43] our_ansi_format(
[ fg( [ bold

                              ,'Hello ~w',
[ world]

                                          ) at /mnt/c/Users/YeGoblynQueenne/Documents/Prolog/metta/metta-wam_win/prolog/metta_lang/metta_testing.pl:252
ERROR:   [41] <user>
ERROR:
ERROR: Note: some frames are missing due to last-call optimization.
ERROR: Re-run your program in debug mode (:- debug.) to get more detail.
   Exception: (52) [ansi_term] ansi_term:rgb([bold, fg('cyan')], _622, _624, _626) ? creep
^  Exception: (43) [user] our_ansi_format([fg([bold, fg('cyan')])], 'Hello ~w', [world]) ? creep
^  Exception: (34) [system] setup_call_cleanup('$toplevel':notrace(call_repl_loop_hook(begin, 0)), '$toplevel':'$query_loop'(0), '$toplevel':notrace(call_repl_loop_hook(end, 0))) ? creep
^  Call: (37) [system] setup_call_cleanup('$notrace'(_43074, _43076), once('$toplevel':call_repl_loop_hook(end, 0)), '$restore_trace'(_43074, _43076)) ? creep
^  Call: (38) [system] sig_atomic(system:'$notrace'(_43074, _43076)) ? creep
^  Exit: (37) [system] setup_call_cleanup(system:'$notrace'(3, -1), system:once('$toplevel':call_repl_loop_hook(end, 0)), system:'$restore_trace'(3, -1)) ? creep
2 ?-
|    .

ERROR: Stream user_input:127:1 Syntax error: Unexpected end of clause

Inspecting the big friendly error dump the problem seems to be that the wrong type ish of data is passed to ansi_format/3 by our_ansi_format/3 at some point. Looking at the definition of our_ansi_format/3 it's obvious this is happening when execution backtracks after the first clause, to the second one, that expects an atom C in its first argument, which it wraps in a list of attributes as expected by ansi_format/3:

our_ansi_format(C, Fmt, Args) :-
    % If Color is not an atom, apply ansi_format directly.
    \+ atom(C), % set_stream(current_output,encoding(utf8)),
    ansi_format(C, Fmt, Args).
our_ansi_format(C, Fmt, Args) :-
    % If Color is atomic, set the foreground color and format the output.
    our_ansi_format([fg(C)], Fmt, Args).

The quick and dirty solution should be obvious but a fuller solution is to implement a proper wrapper for ansi_format/3 that can handle all forms of input accepted by that predicate.