Recursively expand subdictionaries returning dictionary
    dict_expand({1:{2:3}, 4:5}) = {(1,2):3, 4:5}

def dict_expand(d, prefix=None):
    """
    Recursively expand subdictionaries returning dictionary
    dict_expand({1:{2:3}, 4:5}) = {(1,2):3, 4:5}
    """
    result = {}
    for k, v in d.items():
        if isinstance(v, dict):
            result.update(dict_expand(v, prefix=k))
        else:
            result[k] = v

    if prefix is not None:
        result = {make_tuple(prefix) + make_tuple(k): v
                  for k, v in result.items()}
    return result