use strict; use warnings; use signatures;

  sub ( $builder ) {
	return sub ( $n ) {
        return $builder->($builder)->($n);
    };
  }->(
	sub ( $recurse ) {
		return sub ( $n ) {
            if (0 == $n) {
                return 1;
            } else {
				return $n * $recurse->($recurse)->($n - 1);
        	}
        };
    }
  )->(5);