def _enumerate(values):
    """Enumerate possible values."""
    values = list(values)
    if len(values) == 0:
        raise ValueError("Values cannot be empty")
    if len(values) == 1:
        return values[0]
    result = ", ".join(map(repr, values[:-1]))
    if len(values) > 1:
        result = f"{result} or {repr(values[-1])}"
    return result