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- Removing head of list of observations after each reduction step in definition of wp
- Adding support for observations to state_interp and world
- Applying Ralf's suggestions to previous commit (e.g. replacing /\ and -> with unicode characters)

With a pretty proof by Robbert

This restricts CAS to only be able to compare literals with literals, NONEV with NONEV and NONEV with SOMEV for a literal.

* move PROP-envs definitions to environments.v so that we can control them without pulling in coq_tactics
* use reduction-controlled `pm_default` for proofmode accessors

  Also, remove the inconsistency that `wp_expr_eval` succeeds on a goal
  that is not a WP.

We now use the `Maybe` prefix as also used for `Frame`: it indicates
whether progress has been made by stripping of a later or not.

This is to make sure that e.g. `//` in `iEval (rewrite .. // ..)` does
not immediately close the goal by reflexivity.

This reverts commit 913059d2.

This reverts commit 849abb8d, reversing changes
made to 8de0b894.  The merge was done
accidentally, before review happened.