code: func (BitCurve *BitCurve) Double(x1, y1 *big.Int) (*big.Int, *big.Int) {
  	z1 := new(big.Int).SetInt64(1)
  	return BitCurve.affineFromJacobian(BitCurve.doubleJacobian(x1, y1, z1))
  }
docs: // Double returns 2*(x,y)