code: def _branch_name(cls, version):
      
      suffix = version.public[len(version.base_version):]
      components = version.base_version.split('.') + [suffix]
      if suffix == '' or suffix.startswith('rc'):
        # An un-suffixed, or suffixed-with-rc version is a release from a stable branch.
        return '{}.{}.x'.format(*components[:2])
      elif suffix.startswith('.dev'):
        # Suffixed `dev` release version in master.
        return 'master'
      else:
        raise ValueError('Unparseable pants version number: {}'.format(version))
docs: Defines a mapping between versions and branches.
  
      In particular, `-dev` suffixed releases always live on master. Any other (modern) release
      lives in a branch.