code: function mpn_dbl(r, n, l) {
  	var c = 0;
  	for (var i = 0; i < l; ++i) {
  		r[i] = (c = c >>> 31 | n[i] << 1) & 0x7FFFFFFF;
  	}
  	return c >>> 31;
  }
docs: /*
  Doubles the given multi-precision integer n, stores the result into r, and
  returns whether the result overflowed (i.e., returns the "carry-out bit").