docs: def chunkReceived(self, who, chunkNumber, chunkData):
          """
          A chunk was received from the peer.
          """
          def verifyError(error):
              error.trap(VerifyError)
              self.nexus.decreaseScore(who, self.authorities)
          return self.nexus.verifyChunk(self.name,
                                        who,
                                        chunkNumber,
                                        sha.new(chunkData).digest(),
                                        self.authorities).addCallbacks(
              lambda whatever: self.chunkVerified(who, chunkNumber, chunkData),
              verifyError)