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Design pattern:
	Strategy

Description:
	"Define a family of algorithms, encapsulate each one, and make them
	interchangeable. Strategy lets the algorithm vary independently of
	from clients that use it."

This pattern can be used with both classes and prototypes.

The sample implementation uses prototypes and dynamic object predicates
to allow the strategy to be changed dynamically at runtime.

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('behavioral/strategy/loader')).
```

Create a first customer:

```logtalk
customer::new(first, normal_strategy).
```

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Normal billing:

```logtalk
first::add(1.0, 1).
```

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Start Happy Hour:

```logtalk
first::(set_strategy(happy_hour_strategy), add(1.0, 2)).
```

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Create a second customer:

```logtalk
customer::new(second, happy_hour_strategy), second::add(0.8, 1).
```

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First customer pays:

```logtalk
first::print_bill.
```

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End Happy Hour:

```logtalk
second::(set_strategy(normal_strategy), add(1.3, 2), add(2.5, 1), print_bill).
```

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