There are two implementations of a Cache, namely a bounded and an
unbounded variant. This can be elegantly captured in a type class.
In addition to reducing the amount of error-prone code in the
definition of the cache, this version reduces the amount of
error-prone code in usage sites of the cache, as it makes the cache
into an abstract object, so that a calling site cannot distinguish
between cache types. Any decision about what should be cached should
be made through the interface of a cache, rather than at the callsite,
and this is captured by this variant.
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