#!/usr/bin/perl -w
use strict;

print "$_\n" for (<{f}{r,x,y}{e,a,b}{d}>);

__END__
program:  op's post
fred        fred
frad        fxed
frbd        fyed
fxed        frad
fxad        frbd
fxbd        fxad
fyed        fxbd
fyad        fyad
fybd        fybd