def verify_users(g, user):
    """Verify a |user| or all names in the collection |user| as being valid
    Github users."""
    if isinstance(user, str):
        user = [ user ]
    for u in user:
        try:
            g.get_user(u)
        except:
            raise SystemExit('User "%s" does not exist"' % (u,))
    return 1