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This reverts the enforcing part of a44cccbb.

This reverts commit da67461f.

Let us keep the {..} notation for snapshots and shadowed
logical symbols, but go back to transparent access to logical
symbols, at least for now.

- test-ocaml-extraction is back (and fixed to resolve overloaded +)
- exec/extract tests that have been moved to to_port are commented out

x.{f} is allowed and can be used for unary applications
M.{f} is not allowed, use {M.f} instead

It is confusing to use the keyword "label" to define an exception.
Also, the "label L in ..." binds too far and the break point is
not clearly defined.

We do need some syntactic sugar for
  exception X t in try <expr> with X r -> r
(at the very least "break" and "continue"), but labels are not it.

In the surface language, the loop index is always int in
the loop invariant and all annotations and pure terms inside
the loop. If you want to access the original range-typed index,
use "let copy_i = i in" in the program code before your assertion.
Of course, you cannot do that for the loop invariant, which is
what we want.

  Reason for that change: interpret correctly both
  (1) A by B -> C and (2) A -> B by C
  Case (2) is common, and we expect by to take precendence,
  so that A is the context for the proof of B.
  Case (1) is rarer, and is troublesome if by take precendence
  cause (A by B) -> C is almost certainly not what the user
  intended to write. Putting precendence of by/so at the same level as
  the arrow fix this

At this moment, there is no semantics change. This commit only
provides dedicated grammar rules for negated numerals, so that
we do not have to recognize them later.

This allows to import an existing scope to the current namespace.
Not sure if we need this in the surface language, though.

In this way, someone who puts a part of his function
in an abstract block will not have broken "return"s.