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In object-oriented programming, pre- and post-conditions can also use internal
attributes of the object, but then become debugging checks that everything
happens correctly (in the proper state, the proper order, etc...) and cannot
be part of the contract (for external users of the class) since clients cannot
check that the pre-condition is true, because it will not have access to the
internal attributes.

Furthermore, in object-oriented programming, a redefined feature must I<weaken>
the pre-condition of its parent feature and I<strengthen> its post-condition.
It can also keep them as-is.  To fully understand why, it's best to read
Meyer.  Intuitively, it's easy to understand why the pre-condition cannot
be strengthened, nor why the post-condition cannot be weakened: because of dynamic
binding, a caller of r() only has the static type of the object, not its
dynamic type.  Therefore, it cannot know in advance which of the routines will
be called amongst the inheritance tree.

=head2 Common Pitfalls

=over 4

=item *