def get_rel_path(path, base):
    """get relative path, e.g., get_rel_path('abc/de/fg', 'abc') => 'de/fg'
    """
    lb = len(base)
    assert path[:lb] == base
    if len(path) == lb:
        rel_path = ''
    elif path[lb] == '/':
        rel_path = path[lb+1:]
    else:
        rel_path = path[lb:]
    return rel_path