print(Term) :-
    current_prolog_flag(print_write_options, Options), !,
    write_term(Term, Options).
print(Term) :-
    write_term(Term, [ portray(true),
                       numbervars(true),
                       quoted(true)
                     ]).

The print/1 predicate is used primarily through the ~p escape sequence of format/2, which is commonly used in the recipies used by print_message/2 to emit messages.

The classical definition of this predicate is equivalent to the ISO predicate write_term/2 using the options portray(true) and numbervars(true). The portray(true) option allows the user to implement application-specific printing of terms printed during debugging to facilitate easy understanding of the output. See also portray/1 and library(portray_text). SWI-Prolog adds quoted(true) to (1) facilitate the copying/pasting of terms that are not affected by portray/1 and to (2) allow numbers, atoms and strings to be more easily distinguished, e.g., 42, '42' and "42".