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

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

also, export Ty.ty_v_* traversal functions

move old fnT+fnF versions of t_map, t_fold, etc. to a submodule

Rename as little as possible and keep the API.
Make all the necessary checks in Term and Decl.
Remove the duplicate code in Term but keep it elsewhere.
We will factorize the code as we go, without rush.

I love static typing!

- the new syntax for localisation "labels" in Why is as follows:

    goal Toto #"file" line bchar echar#     - after an ident
    #"file" line bchar echar" (A and B)     - before a term/fmla

- the new syntax for buffer relocation is as follows:

    ##"file" line char##

the verification algorithm must always terminate and be reasonably
performant in practice, but its worst-case complexity is unknown
and probably exponential. What is quite easy when there is only
one recursive definition, becomes difficult when there is a group
of mutually recursive definitions. An educated discussion would
be highly appreciated.

BTW, I had to convert a couple of recursive "logic"s on integers
into an abstract "logic" + axiom. Pretty much all of them supposed
that the argument was non-negative, and thus were non-terminating!

= replaced by ls_equal in core/pretty (was discovered thanks to a Invalid_argument(equal: functional value)