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Design pattern:
	Marker interface

Description:
	Marker interfaces, also known as tagging interfaces, allows
	associating meta-data with a hierarchy of objects. Typically,
	marker interfaces are empty with its presence implying that
	an object implements some functionality or have some property.

This pattern can be used with both classes and prototypes.

This pattern is not described in the GoF book but is common practice
in some languages (notably, Java). See e.g. the following Wikipedia
page for details and references:

https://en.wikipedia.org/wiki/Marker_interface_pattern

Given Logtalk support for protocols (interfaces), this is a trivial
pattern to implement. The reflection API provides built-in predicates
that allows to test if an object (or a category) implements or conforms
to a given protocol. It is also possible to add a marker to an existing
object at runtime using a complementing category (aka hot patching).

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

```logtalk
%versions
```

Start by loading the design pattern sample implementation:

```logtalk
logtalk_load(design_patterns('structural/marker_interface/loader')).
```

Check for the marker interface:

```logtalk
%%table
conforms_to_protocol(Entity, marker).
```

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