$S = 'o';
    
    for (1..16) {
        $S .= 'p';                  # S  -> Sp
        @processing = ($S);
    
        $S =~ s/op/i/;              # op -> i
        $S =~ s/ip/c/;              # ip -> c
        push @processing, $S;
    
        while ($S =~ /c/) {
            $S =~ s/oc/io/;         # oc -> io
            $S =~ s/ic/co/;         # ic -> co
            $S =~ s/^c/io/;         # cS -> ioS
            push @processing, $S;
        }
        
        @out = map { sprintf ("%6s", $_) } @processing;
        printf ("%-44s == %6s\n", join (" ->", @out), $S);
    }