This became broken after the nested iSpecialize MR.

It now supports the specialization pattern `(H spat1 .. spatn)`, which first
recursively specializes the hypothesis `H` using the specialization patterns
`spat1 .. spatn`.

The continuation is called with a Boolean indicating whether the
hypothesis was in the intuitionistic context or not.

Split it up into more logical parts.

- More consistent indentation.
- Mark new subgoals as comments.

This is to avoid confusion with those of selection patterns.

This lemma is similar to `later_ownM`.

This lemma allows one to get the witness out of a later, without having
to use `later_car`, i.e. in a way that works in the on-paper version of
the logic.