code: func MustAbsInt64(n int64) int64 {
  	if n == -1<<63 {
  		panic("value out of range")
  	}
  	y := n >> 63       // y ← x ⟫ 63
  	return (n ^ y) - y // (x ⨁ y) - y
  }
docs: // MustAbsInt64 returns the absolute value of an int64.