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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

thus we gain more goals than we lose

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).

until we understand why keeping them makes nightlies regress