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# engines - fluents

This example requires support for both threads and coroutining. Currently it
runs on ECLiPSe and SWI-Prolog. It should run also on XSB and YAP if and when
these systems bugs with coroutining and/or threads get fixed.

This folder contains examples of fluents implemented using threaded engines.
Fluents are described in the paper:

```bibtex
@inbook{Tarau2000,
	author="Tarau, Paul",
	editor="Lloyd, John and Dahl, Veronica and Furbach, Ulrich and Kerber, Manfred and Lau, Kung-Kiu and Palamidessi, Catuscia and Pereira, Lu{\'i}s Moniz and Sagiv, Yehoshua and Stuckey, Peter J.",
	chapter="Fluents: A Refactoring of Prolog for Uniform Reflection and Interoperation with External Objects",
	title="Computational Logic --- CL 2000: First International Conference London, UK, July 24--28, 2000 Proceedings",
	year="2000",
	publisher="Springer Berlin Heidelberg",
	address="Berlin, Heidelberg",
	pages="1225--1239",
	isbn="978-3-540-44957-7",
	doi="10.1007/3-540-44957-4_82",
	url="http://dx.doi.org/10.1007/3-540-44957-4_82"
}
```

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

```logtalk
%versions
```

Load the example:

```logtalk
logtalk_load(fluents(loader)).
```

<!--
true.
-->

Get answers from the fluent:

```logtalk
fluents::next(N1).
```

<!--
N1 = 1.
-->

```logtalk
fluents::next(N2).
```

<!--
N2 = 2.
-->

```logtalk
fluents::next(N3).
```

<!--
N3 = 3.
-->

After exhausting the fluent answers, subsequent queries fail
until the threaded engine implementing the fluent is destroyed:

```logtalk
\+ fluents::next(_).
```

<!--
true.
-->

```logtalk
\+ fluents::next(_).
```

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true.
-->
