@classmethod
def _merge(cls, default, user):
    """Append user options to default options. Return yala section."""
    section = cls._CFG_SECTION
    merged = default[section]
    if section not in user:
        return merged
    user = user[section]
    for key, value in user.items():
        if key in merged:
            merged[key] += ' ' + value
        else:
            merged[key] = value
    return merged
