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This file is part of Logtalk <https://logtalk.org/>  
Copyright 1998-2021 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.
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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.
