code: func int64Add(a, b int64) (int64, bool) {
  	c := a + b
  	switch {
  	case a > 0 && b > 0:
  		if c < 0 {
  			return 0, false
  		}
  	case a < 0 && b < 0:
  		if c > 0 {
  			return 0, false
  		}
  		if a == mostNegative && b == mostNegative {
  			return 0, false
  		}
  	}
  	return c, true
  }
docs: // int64Add returns a+b, or false if that would overflow int64.