def _join_ver(ver_tuple):
    """Join the given version-tuple, inserting '.' as appropriate.
    
        >>> _join_ver( ('4', '1', '0') )
        "4.1.0"
        >>> _join_ver( ('1', '3', 'a', '2') )
        "1.3a2"
    """
    def isint(s):
        try:
            int(s)
        except ValueError:
            return False
        else:
            return True

    dotted = []
    for bit in ver_tuple:
        if dotted and isint(dotted[-1]) and isint(bit):
            dotted.append('.')
        dotted.append(str(bit))
    return ''.join(dotted)