I know we don't use it.  Stating theorems also serves to document things, and IMHO this one is informative.  It also costs us nothing.

This fixes a bug in 916ff44a causing proof mode notations not being
pretty printed.

Concretely, when execution of any of the wp_ tactics does not yield
another wp, it will make sure that a view shift is kept. This
behavior was already partially there, but now it is hopefully more
consistent.

This tweak allows us to declare pvs as an instance of FromPure (it is not
an instance of IntoPure), making some tactics (like iPureIntro and done)
work with pvs too.

This reverts commit 4c056f5e.

We are now using the prefixes Into, From, and Is (the first two are
inspired by the names of some traits in the Rust stdlib), and hopefully
doing that consistenly.

This is very experimental. It should now deal better with stuff like:

  match x with .. end = match y with .. end

In case there is a hypothesis H : R x y, it will try to destruct it.

This is more consistent with the proofmode, where we also call it pure.

This is more natural, match should be used in user code, not case.

Fun test case to play arround with abstract predicates.

Fixes issue #20.