$S = 'opppppppp';               # (S  -> Sp) x 8
    
    while ($S =~ /p/) {
        @processing = ($S);
        
        $S =~ s/op/i/;              # op -> i
        push @process, $S;
        $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 ("%9s", $_) } @processing;
        print join (" ->", @out), "\n";
    }