def register(key)
      return nil if bibliography.nil?

      k = key.dup
      k.succ! while bibliography.has_key?(k)
      bibliography.entries[k] = self
      k
    end
def register(key)
      return nil if bibliography.nil?

      k = key.dup
      k.succ! while bibliography.has_key?(k)
      bibliography.entries[k] = self
      k
    end
Registers this Entry in the associated Bibliographies entries hash.
 This method may change the Entry's key, if another entry is already
 registered with the current key.

 Returns the key or nil if the Entry is not associated with a Bibliography.