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Useful to break out of the loops:

  label Break in
  while ... do
    label Continue in
    ... raise Break ...
    ... raise Continue ...
  done

When a label is put over a non-unit expression,
raise acts as return:

  label Return in
  if ... then raise Return 42; 0

Also, "return <expr>" returns from the innermost function.
This includes abstract blocks, too, so if you want to return
across an abstract block, you should rather use a label at
the top of the main function. TODO/FIXME: maybe we should
let "return" pass across abstract blocks by default, to
avoid surprises?

One shortcoming of the labels-as-exceptions is that they cannot
be used to transmit tuples with ghost elements, nor return ghost
values from non-ghost expressions. A local exception with an
explicit mask should be used instead. Similarly, to return
a partially ghost value from a function, it must have have
its mask explicitly written (which is a good practice anyway).
We cannot know the mask of an expr before we construct it,
but in order to construct it, we need to create the local
exceptions first.

Another caveat is that while it is possible to catch an exception
generated by a label, you should avoid to do so. We only declare
the local exception if the expression under the label _raises_
the exception, and thus the following code will not typecheck:

  label X in (try raise X with X -> () end)

Indeed, the expression in the parentheses does not raise X,
and so we do not declare a local exception X for this label,
and so the program contains an undeclared exception symbol.

current syntax is

    exception Return (int, ghost bool) in
    ...
    try
      ...
      raise Return (5, false)
      ...
    with
      Return (i, b) -> ...
    ...

These exceptions can carry mutable and non-monomorphic values.
They can be raised from local functions defined in the scope
of the exception declaration.

Since local exceptions can carry results with type variables
and regions, we need to freeze them in the function signatures.

avoid using logical symbols in programs in a few places

I know that extraction and execution are much more fun than
the boring "can we even parse WhyML at all?" question, but
you see, if our parsing/typechecking is broken, then the fun
stuff is broken, too. And I really prefer to get the error
messages about the typechecking being broken from the typechecking
part of the bench, and not from the test-api or extraction part.
Merci pour votre compréhension.

Aesthetics is a harsh mistress.

This is a prototype version that requires no additional annotation.
In addition to the existing rules of scoping:

  - it is forbidden to declare two symbols with the same name
    in the same scope ("everything can be unambiguously named"),

  - it is allowed to shadow an earlier declaration with a newer one,
    if they belong to different scopes (e.g., via import),

we define one new rule:

  - when an rsymbol rs_new would normally shadow an rsymbol rs_old,
    but (1) both of them are either
      - binary relations: t -> t -> bool,
      - binary operators: t -> t -> t, or
      - unary operators:  t -> t
    and (2) their argument types are not the same,
    then rs_old remains visible in the current scope.
    Both symbols should take non-ghost arguments and
    return non-ghost results, but effects are allowed.
    The name itself is not taken into account, hence
    it is possible to overload "div", "mod", or "fact".

Clause (1) allows us to perform type inference for a family of rsymbols,
using an appropriate generalized signature.

Clause (2) removes guaranteed ambiguities: we treat this case as
the user's intention to redefine the symbol for a given type.

Type inference failures are still possible: either due to
polymorphism (['a -> 'a] combines with [int -> int] and
will fail with the "ambiguous notation" error), or due
to inability to infer the precise types of arguments.
Explicit type annotations and/or use of qualified names
for disambiguation should always allow to bypass the errors.

Binary operations on Booleans are treated as relations, not as
operators. Thus, (+) on bool will not combine with (+) on int.

This overloading only concerns programs: in logic, the added rule
does not apply, and the old symbols get shadowed by the new ones.

In this setting, modules never export families of overloaded symbols:
it is impossible to "use import" a single module, and have access to
several rsymbols for (=) or (+).

The new "overloading" rule prefers to silently discard the previous
binding when it cannot be combined with the new one. This allows us
to be mostly backward compatible with existing programs (except for
the cases where type inference fails, as discussed above).

This rule should be enough to have several equalities for different
program types or overloaded arithmetic operations for bounded integers.
These rsymbols should not be defined as let-functions: in fact, it is
forbidden now to redefine equality in logic, and the bounded integers
should be all coerced into mathematical integers anyway.

I would like to be able to overload mixfix operators too, but
there is no reasonable generalized signature for them: compare
"([]) : map 'a 'b -> 'a -> 'b" with "([]) : array 'a -> int -> 'a"
with "([]) : monomap -> key -> elt". If we restrict the overloading
to symbols with specific names, we might go with "'a -> 'b -> 'c" for
type inference (and even "'a -> 'b" for some prefix operators, such
as "!" or "?"). To discuss.

As we have to implement the "HERE" and "PARENT" qualifiers anyway,
allowing this shadowing lets us accept more programs without
compromising the "everything is unambigously nameable" invariant.

  (+ élimination de la preuve Coq)