def normalize_pred_string(predstr):
    """
    Normalize the predicate string *predstr* to a conventional form.

    This makes predicate strings more consistent by removing quotes and
    the `_rel` suffix, and by lowercasing them.

    Examples:
        >>> normalize_pred_string('"_dog_n_1_rel"')
        '_dog_n_1'
        >>> normalize_pred_string('_dog_n_1')
        '_dog_n_1'
    """
    tokens = [t for t in split_pred_string(predstr)[:3] if t is not None]
    if predstr.lstrip('\'"')[:1] == '_':
        tokens = [''] + tokens
    return '_'.join(tokens).lower()