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This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2023 Paulo Moura <pmoura@logtalk.org> SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

To load this example and for sample queries, please see the SCRIPT.txt file.

This example illustrates how to use the optional terms library to decouple data acquisition, which must be able to represent optional values in the data, from data processing, which decides how to handle those values and their absence. The use of optionals terms avoids the often problematic solution of using special values to represent the absence of optionals values. The optional terms library predicates allow handling the optional terms without the need to use if-then-else or cut control constructs.

For a description of this example, please see the comments in the books.lgt source file.

See also the missing_data and cascade examples.