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- No more "and", "or", "implies", "iff", and "~".
  Use "/\", "\/", "->", "<->", and "not" instead.

- No more "logic". Use "function" or "predicate".

experimental distance of terms dropped

in particular, there is no more need to define intermediate
sorts or to close the set of protected types wrt subtyping

You must run ./config.status after this commit

- provide a method to mark proof attempts as obsolete
  (thus, we can replay a saved tree even if the source
  file hasn't changed)
- cleaning applies to a subtree (like transformations
  and provers), not only at the first level
- obsolete proof attempts do not count as successuful

also apply transformations to the proved subgoals if "all subgoals"
is selected (just as we do it for provers)

- create_env_of_loadpath is now provided in Env instead of Lexer
- find_channel functions now depend on format to determine the
  suitable extensions

also preserve proposition names in split_premise when possible

What was its purpose in the first place? Integers are protected
in Simplify anyway and then we can simply forget the difference
between the infinite sorts (as we do in encoding_tptp).

This permits using them in transformations, as they also
preserve the term structure.

Export Term.t_ty_map and Term.t_app_map. We can generalize
t_s_map to cover t_app_map, but this requires computing the
list of types for every application, which we rarely need.

which gives us a cheap test for recursive defitions.
We could also make an effort to ignore the conclusions
in inductive predicate definitions but it's probably
not worth it.

also, export Ty.ty_v_* traversal functions