code: function fp_sub(r, n1, n2, p, l) {
  	if (mpn_sub(r, n1, n2, l)) {
  		mpn_add(r, r, p, l);
  	}
  	return r;
  }
docs: /*
  Subtracts two multi-precision integers n1 and n2 in the finite field of
  order p, stores the difference n1 - n2 into r, and returns r.