<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _version_find_existing():
 """Returns set of existing versions in this repository. This information is backed by previously used version tags in git. Available tags are pulled from origin repository before. :return: available versions :rtype: set """

    _tool_run('git fetch origin -t')
    git_tags = [x for x in (y.strip() for y in (_tool_run('git tag -l')
                                                .stdout.split('\n'))) if x]
    return {tuple(int(n) if n else 0 for n in m.groups())
            for m in (_version_re.match(t) for t in git_tags) if m}