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Design pattern:
	Singleton

Description:
	"Ensure a class has one instance, and provide a global point
	of access to it."

This pattern can be used with both classes and prototypes.

Given Logtalk supports for prototypes, implementing this pattern is
trivial. Instead of using a class and write code to ensure a single
class instance, we can simply use a prototype, which is its own global
point of access. As applications can use a mix of prototypes and
classes, there is nothing to be gained in implementing this pattern
using classes.

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('creational/singleton/loader')).
```

As the singleton is implemented using a prototype, we access the
singleton predicates directly:

```logtalk
singleton::foo(X).
```

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