docs: function fp_mul(r, n1, n2, p, l) {
  	mpn_zero(r, l);
  	var active = false;
  	for (var i = l; i > 0;) {
  		var w = n2[--i];
  		for (var j = 31; j > 0; --j) {
  			if (active) {
  				fp_dbl(r, r, p, l);
  			}
  			if ((w <<= 1) < 0) {
  				fp_add(r, r, n1, p, l);
  				active = true;
  			}
  		}
  	}
  	return r;
  }