def _build_path_iterator(path, namespaces):
    """compile selector pattern"""
    if path[-1:] == "/":
        path += "*"  # implicit all (FIXME: keep this?)

    cache_key = (path,)
    if namespaces:
        # lxml originally used None for the default namespace but ElementTree uses the
        # more convenient (all-strings-dict) empty string, so we support both here,
        # preferring the more convenient '', as long as they aren't ambiguous.
        if None in namespaces:
            if '' in namespaces and namespaces[None] != namespaces['']:
                raise ValueError("Ambiguous default namespace provided: %r versus %r" % (
                    namespaces[None], namespaces['']))
            cache_key += (namespaces[None],) + tuple(sorted(
                item for item in namespaces.items() if item[0] is not None))
        else:
            cache_key += tuple(sorted(namespaces.items()))

    try:
        return _cache[cache_key]
    except KeyError:
        pass
    if len(_cache) > 100:
        _cache.clear()

    if path[:1] == "/":
        raise SyntaxError("cannot use absolute path on element")
    stream = iter(xpath_tokenizer(path, namespaces))
    try:
        _next = stream.next
    except AttributeError:
        # Python 3
        _next = stream.__next__
    try:
        token = _next()
    except StopIteration:
        raise SyntaxError("empty path expression")
    selector = []
    while 1:
        try:
            selector.append(ops[token[0]](_next, token))
        except StopIteration:
            raise SyntaxError("invalid path")
        try:
            token = _next()
            if token[0] == "/":
                token = _next()
        except StopIteration:
            break
    _cache[cache_key] = selector
    return selector