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In programs, but also in pure theories, it is not safe to compare
arbitrary types. For example, if we have a record with ghost fields,
a comparison may produce different results before and after ghost
code elimination. Even for pure types like 'map' or 'set', it is
unlikely that the result of logical equality will be the same as
the result of OCaml structural equality on the implemented type.

This commit makes the first step towards fixing this issue.
We proceed in the following way:

1. Every lsymbol (pure function or predicate symbol) carries
   a subset of type variables of its signature, called "opaque
   type variables". By marking a type variable 'a opaque in an
   lsymbol's signature, the user guarantees that this lsymbol
   can be implemented without ever comparing values of type 'a.
   In other words, this is a promise not to break into a type
   variable.

   The corresponding syntax is: "predicate safe (x y : ~'a)".

   All type variables in undefined symbols are non-opaque,
   unless annotated otherwise. Non-opaque is the default
   to keep the change conservative.

   Opacity of type variables in defined symbols is inferred
   from the definition. If the definition violates a given
   opacity annotation, an exception is raised. Notice that
   we only check definitions in _theory_ declarations. One
   can define an lsymbol in a _task_ in a way that violates
   opacity. We cannot forbid it, because various elimination
   transformations would replace safe operations (such as
   matching) with equalities. This is not a problem, since in
   the pure logical realm of provers opacity is not required
   One exception would be Coq, whose transformation chain must
   never perform such operations.

   All type variables in inductive predicates are non-opaque.
   Indeed, we can redefine equality via an inductive predicate.
   [TODO: find safe forms of inductive definitions and implement
   more reasonable restrictions.]

   All type variables in constructors and field symbols are opaque.

   It is forbidden to instantiate an opacity-preserving symbol
   with an opacity-breaking one in a clone substitution.

2. Similar type variable tracking is implemented for program symbols.
   Type variables in the signature of a "val" are non-opaque unless
   annotated otherwise. Opacity of type variables in defined symbols
   is inferred from the definition, and an exception is raised, if
   a given annotation is violated.

   The internal mechanism of tracking is different: the "eff_compar"
   field in effects contains the type variables that occur under
   equality or any other opacity-breaking operation. In this respect,
   our API is inconsistent between lsymbols and psymbols: the former
   asks for the opaque tvsymbols, the latter requires us to fill the
   spec with "comparison effects" for the non-opaque ones. [TODO:
   add the "~opaque" argument to create_psymbol and make the WhyML
   core fill the effect under the hood.]

   Every time an lsymbol or a psymbol is applied in a program,
   we check the substitution into its signature's type variables.
   If a non-opaque type variable is instantiated with a program type,
   an exception is raised. [TODO: be more precise and reject only
   types with ghost and model components - being mutable, private,
   or carrying an invariant doesn't conflict with equality.]

   Notice that we do not allow to compare program types even in
   the ghost code. This is not needed if we only consider the
   problems of the code extraction, but _might_ be necessary,
   if we also want to protect Coq realisations (see below).

This commit fixes the immediate problem of breaking the ghost
guarantees when equality or some other opacity-breaking lsymbol
is applied in a program to a type with ghost or "model" parts.

This leaves the problem of code extraction for programs that
compare complex types such as maps or sets (Coq driver is
affected by this, too, I guess). The next step is to provide
annotations for problematic type constructors. A declaration
"type ~map 'a 'b" would mean "logical equality on this type
is likely to be different from the structural equality on any
implementation of this type - therefore do not apply equality
to it: neither in programs (because this can't be implemented),
nor in pure functions (because they are extracted, too, and
because this can't be realized with Leibniz equality in Coq)."
[TODO: discuss and implement.]

[TODO: mb choose better term for "opaque" and notation for ~'a.]

Now, there are three kinds of types used in WhyML API:
  ity -- the type of first-order values, "i" stands for "individual"
  aty -- the type of higher-order values, "a" stands for "arrow"
  vty -- the sum of the previous two, "v" stands for "value"

We should probably rename the VTvalue constructor, since it carries
an ity, and not a vty. And I would gladly rename ity to something
more appropriate, too.

Instead, we now store the ghostness boolean directly
in pvsymbols, pssymbols, ppatterns, and exprs. We thus
remove one level of indirection, and vty_value becomes
a one-field record which we remove in the next commit.

This simplifies semantics, makes a lot of checks unnecessary,
and makes us compatible with OCaml.

the Set signature is moved to a separate Extset module,
instead of being included into Extmap.S. The module hierarchy
becomes simpler, we don't shadow  OCaml standard modules anymore,
and the bug #15270 is fixed, too.

+ rename Mlw_ty.ity_pure to ity_immutable

export Why3's extended hash tables as Stdlib.XHashtbl
in order to keep OCaml's Hashtbl accessible. Unlike Stdlib.Map
which fully covers the OCaml's interface, Stdlib.XHashtbl does
not provide polymorphic hash tables.

Util now is a small module containing misc functions.

+ fix a bug in "compare"

+ rename Debug.Opt to Debug.Args to avoid conflicts

+ create AUTHORS file
+ fix the linking exception in LICENSE
+ update the "About" in IDE
+ remove the trailing whitespace
+ inflate my scores at Ohloh

+ do not build src/programs/
+ build the library in lib/why3/
+ fix use_api.ml for the case when Alt-Ergo is not present
+ distribute lib/coq/BuiltIn.v
+ rename src/config to src/why3config
+ rename src/bench to src/why3bench
+ move src/config.ml to src/util/

- get rid of val_decl, the psymbol/pvsymbol is enough
- get rid of vtv_vars, keep the varset in pvsymbol
- include fresh regions in pd_news (will serve for clone)

Merge specifications into program types, as JCF intended.