how does print-to-codes work?

I have made a procedure that tidies up the printing of floats and it works OK 
if consulted as portray/1 for the built in print predicate. But if I use 
print_to_codes the result of portray/1 is still sent to the console. Here's 
an example...

$ gprolog
GNU Prolog 1.2.14
By Daniel Diaz
Copyright (C) 1999-2002 Daniel Diaz
| ?- print(2.92).
2.9199999999999999

yes
| ?- print_to_codes(A, 2.92).

A = [50,46,57,49,57,57,57,57,57,57,57,57,57,57,57,57,57,57]

yes
| ?- consult(nicefloat).
compiling /win98/Program Files/Simile/Develop/Tests/nicefloat.pl for byte 
code...
/win98/Program Files/Simile/Develop/Tests/nicefloat.pl compiled, 6 lines read 
- 1503 bytes written, 8 ms

yes
| ?- portray(2.92).
2.92

yes
| ?- print(2.92).
2.92

yes
| ?- print_to_codes(A, 2.92).
2.92

A = []

yes
| ?-

portray(F) :-
	float(F),
	format_to_codes(Fs, "~6f", [F]),
	append(Ns, Os, Fs),
	\+ (member(Ch, Os), \+ member(Ch, "0.")), !,
	format("~s",[Ns]).