def __init__(self, vcs, rev=None, extra_args=None):<EOL>
if extra_args is None:<EOL><INDENT>extra_args = []<EOL><DEDENT>self.extra_args = extra_args<EOL>self.rev = rev<EOL>self.vcs = vcs<EOL>
Args:
  vcs: a VersionControl object.
  rev: the name of the revision to install.
  extra_args: a list of extra options.