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Design pattern:
	Visitor

Description:
	"Represent an operation to be performed on the elements of an
	object structure. Visitor lets you define a new operation without
	changing the classes of the elements on which it operates."

This pattern can be used with both classes and prototypes.

This design pattern can be trivially implemented by defining a suitable
meta-predicate that walks the structure and applies a user defined
closure to each element. When the object defining the structure doesn't
provide such a meta-predicate, a complementing category can define and
add it to the object. Both cases are illustrated in the sample code.

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/visitor/loader')).
```

Print all car components by calling the `visitor/1` meta-predicate with
a closure (use a lambda expression so that we can print a new-line after
each component):

```logtalk
sedan::visitor([Component]>>(write(Component),nl)).
```

<!--
engine(diesel)
wheel(front_left)
wheel(front_right)
wheel(rear_right)
wheel(rear_left)
wheel(left_door)
wheel(right_door)
body(station_wagon)

true.
-->

Use the `visitor/1` meta-predicate that is added using hot-patching:

```logtalk
sedan::alt_visitor([Component]>>(write(Component),nl)).
```

<!--
engine(diesel)
wheel(front_left)
wheel(front_right)
wheel(rear_right)
wheel(rear_left)
wheel(left_door)
wheel(right_door)
body(station_wagon)

true.
-->

Use the standard `setof/3` meta-predicate to construct a list of all
component types:

```logtalk
setof(
		Type,
		Component^Arity^(sedan::component(Component), functor(Component,Type,Arity)),
		Types
     ).
```

<!--
Types = [body, engine, wheel].
-->
