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- provide several helper functions in Task
- Pretty.print_named_task will also print Use and Clone decls

This will allow transformations to apply use and clone.
Also, this will allow to simplify the driver structure.
Memoisation modulo env/clone/use requires some further 
adaptation (but this would be needed anyway).

  as it is most often ignored
- check for empty map in t_subst/f_subst
- bugfix: don't forget nested match statements in Decl and Compile_match

  (TODO gestion des commentaires, des match, forall, ...)
   - call_provers : Utilisation de Unix.create_process au lieu de Sys.command

  move this implementation to eliminate_let.
- initial commit of eliminate_definition : the goal is
  to translate definitions by match into series of axioms.

- check pattern well-formedness in Pattern
- add arguments to exceptions in Ty and Term

(unusable so far, must eliminate match statements, too)