And make constants P for which we do not want of_val P to reduce Opaque.

Since strip_later is doing a good job stripping laters in the conclusion,
these tactics are thus no longer needed. Also, wp_finish now properly
converts the result in a primitive viewshift in case it is not a weakestpre.

Thanks to Amin Timany for the suggestion.

Tactics like wp_proj should always solve all to_val side-conditions. The
tactic wp_done is used to handle these in a uniform way.

We no longer abuse empty strings for anonymous binders. Instead, we
now have a data type for binders: a binder is either named or
anonymous.

It is based on type classes and can it be tuned by providing
instances, for example, instances can be provided to mark that
certain expressions are closed.

this makes it slightlymore annoying to use because we have to elliminate the box. one more reason to have a proof mode ;-)