Given a diff_hist as created, appended by append_diff_hist, yield the
    versions of the object start with curr_obj and working backwards in time.
    Each instance yielded is of the form (obj, date-string) where obj is the
    JSON version of the object created by applying a diff in the
    diff history and date-string is a string representing the date/time that
    the diff was taken

def parse_diff_hist(curr_obj, diff_hist):
    """Given a diff_hist as created, appended by append_diff_hist, yield the
    versions of the object start with curr_obj and working backwards in time.
    Each instance yielded is of the form (obj, date-string) where obj is the
    JSON version of the object created by applying a diff in the
    diff history and date-string is a string representing the date/time that
    the diff was taken"""
    curr_obj, diff_hist = _norm_json_params(curr_obj, diff_hist)

    yield (json.dumps(curr_obj), None)

    last_obj = curr_obj
    for one in reversed(diff_hist):
        last_obj = record_patch(last_obj, one['diff'])
        yield json.dumps(last_obj), one['diff_date']