Blurb::
A model whose responses are computed through the use of a sub-iterator
Description::
Instead of appealing directly to a primary interface, a nested model
maps variables to responses by executing a secondary iterator, or a
"sub-iterator".
In other words, a function evaluation of the primary study consists of a
solution of an entire secondary study - potentially many secondary
function evaluations.

The sub-iterator in turn operates on a
sub-model. The sub-iterator responses may be combined with non-nested
contributions from an optional interface specification.

A * ``sub_method_pointer*`` must be provided
in order to specify the method block describing the sub-iterator.
The remainder of the model is specified under that keyword.

A \b ``optional_interface_pointer`` points
to the interface specification and
``optional_interface_responses_pointer`` points to a responses
specification describing the data to be returned by this interface.
This interface is used to provide non-nested data, which is then
combined with data from the nested iterator using the
``primary_response_mapping``. (See the discussions for the
``sub_method_pointer`` and ``optional_interface_pointer`` keywords.)
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