sub common_suffix {
        my $comm = shift @_;
        while ($_ = shift @_) {
            $_ = substr($_,-length($comm)) 
                if (length($_) > length($comm));
            $comm = substr($comm,-length($_)) 
                if (length($_) < length($comm));
            if (( $_ ^ $comm ) =~ /(\0*)$/) {
                $comm = substr($comm, -length($1));
            } else {
                return undef;
            }
        }
        return $comm;
    }
    
    
    sub common_prefix {
        my $comm = shift @_;
        while ($_ = shift @_) {
            $_ = substr($_,0,length($comm)) 
                if (length($_) > length($comm));
            $comm = substr($comm,0,length($_)) 
                if (length($_) < length($comm));
            if (( $_ ^ $comm ) =~ /^(\0*)/) {
                $comm = substr($comm,0,length($1));
            } else {
                return undef;
            }
        }
        return $comm;
    }