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(though it will be simpler and cheaper to use references)

This reverts commit 9809329892bb9637a0137d330ecd08a16ede2d93.

The relevant hash-tables are weak both in keys and values

- move exception reporting for Ty, Pattern, Term, and Decl
  to Pretty, as it occasionnaly requires some pretty-printing

I will always compile before committing
I will always compile before committing
I will...

like f_map_sign should only be applied from specialized
recursive functions and these funcitons will usually supply
the polarity argument.

It's not more expressive but much nicer than epsilon.

  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

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

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