def _version_difference_str(authority, consensus_versions, vote_versions):
  """
  Provide a description of the delta between the given consensus and vote
  versions. For instance...

    moria1 +1.0.0.1-dev -0.0.8.6 -0.0.8.9
  """

  consensus_versions = set(consensus_versions)
  vote_versions = set(vote_versions)

  msg = authority

  for extra_version in vote_versions.difference(consensus_versions):
    msg += ' +%s' % extra_version

  for missing_version in consensus_versions.difference(vote_versions):
    msg += ' -%s' % missing_version

  return msg