code: func (e *gfP2) MulXi(a *gfP2, pool *bnPool) *gfP2 {
  	// (xi+y)(i+3) = (9x+y)i+(9y-x)
  	tx := pool.Get().Lsh(a.x, 3)
  	tx.Add(tx, a.x)
  	tx.Add(tx, a.y)
  
  	ty := pool.Get().Lsh(a.y, 3)
  	ty.Add(ty, a.y)
  	ty.Sub(ty, a.x)
  
  	e.x.Set(tx)
  	e.y.Set(ty)
  
  	pool.Put(tx)
  	pool.Put(ty)
  
  	return e
  }
docs: // MulXi sets e=ξa where ξ=i+9 and then returns e.