code: function fp_dbl(r, n, p, l) {
  	if (mpn_dbl(r, n, l) || mpn_cmp(r, p, l) >= 0) {
  		mpn_sub(r, r, p, l);
  	}
  	return r;
  }
docs: /*
  Doubles the given multi-precision integer n in the finite field of order p,
  stores the result into r, and returns r.