How to define a dummy/placeholder predicate in GNU Prolog -

i have prolog file following structure:

% library section %  foo(x) :- bar(x);           baz(x).  % user data section %  % e.g. bar(charlie). 

the user data of file intended allowed extended user default contains nothing. causes query foo(x). fails because bar/1 , baz/1 not defined.

i have tried defining them placeholder values (i.e. bar(none).) gnu prolog complains discontiguous predicates when user data added bottom of file.

is there way define dummy/placeholder version of bar/1 , baz/1 foo(x). not fail , other lines containing bar , baz can added bottom of file?

if understand question, want have along lines of:

 ask_bar :-     % user input     assertz(bar(input)).  foo(x) :-     bar(x). 

if indeed problem, have 2 options:

first one: declare bar/1 dynamic predicate:

:- dynamic(bar/1). 

(this directive, type :- @ beginning of line.)

second one: in program, before reference bar/1, call predicate retractall/1, this:

 main :-     retractall(bar(_)),     %.... 

this delete bars database, and declare bar/1 dynamic.