def METHOD_NAME(op):
    """Filter which includes only UDFs from expression tree."""
    if type(op) in BigQueryExprTranslator._rewrites:
        op = BigQueryExprTranslator._rewrites[type(op)](op)
    if isinstance(op, operations.BigQueryUDFNode):
        result = op
    else:
        result = None
    return lin.proceed, result