<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def make_vcs_requirement_url(repo_url, rev, project_name, subdir=None):
 """ Return the URL for a VCS requirement. Args: repo_url: the remote VCS url, with any needed VCS prefix (e.g. "git+"). project_name: the (unescaped) project name. """

    egg_project_name = pkg_resources.to_filename(project_name)
    req = '{}@{}#egg={}'.format(repo_url, rev, egg_project_name)
    if subdir:
        req += '&subdirectory={}'.format(subdir)

    return req