code: func (r *PortAllocator) Has(port int) bool {
  	ok, offset := r.contains(port)
  	if !ok {
  		return false
  	}
  
  	return r.alloc.Has(offset)
  }
docs: // Has returns true if the provided port is already allocated and a call
  // to Allocate(port) would fail with ErrAllocated.