code: func (r *PortAllocator) contains(port int) (bool, int) {
  	if !r.portRange.Contains(port) {
  		return false, 0
  	}
  
  	offset := port - r.portRange.Base
  	return true, offset
  }
docs: // contains returns true and the offset if the port is in the range, and false
  // and nil otherwise.