code: def _jar_id(jar):
      
      if jar.rev:
        return '{0}:{1}:{2}'.format(jar.org, jar.name, jar.rev)
      else:
        return '{0}:{1}'.format(jar.org, jar.name)
docs: Create a string identifier for the IvyModuleRef key.
      :param IvyModuleRef jar: key for a resolved jar
      :returns: String representing the key as a maven coordinate