code: function mpn_sub(r, n1, n2, l) {
  	var c = 0;
  	for (var i = 0; i < l; ++i) {
  		r[i] = (c = (c >> 31) + n1[i] - n2[i]) & 0x7FFFFFFF;
  	}
  	return c >> 31;
  }
docs: /*
  Subtracts two multi-precision integers n1 and n2, stores the difference n1 -
  n2 into r, and returns whether the subtraction underflowed (i.e., returns
  the "borrow-out bit").