code: func (alloc *allocator) Alloc(tableID int64) (int64, error) {
  	if tableID == 0 {
  		return 0, errInvalidTableID.GenWithStack("Invalid tableID")
  	}
  	alloc.mu.Lock()
  	defer alloc.mu.Unlock()
  	if alloc.isUnsigned {
  		return alloc.alloc4Unsigned(tableID)
  	}
  	return alloc.alloc4Signed(tableID)
  }
docs: // Alloc implements autoid.Allocator Alloc interface.