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Check ground models with Dolmen #790

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@Halbaroth Halbaroth commented Aug 24, 2023

Please do not merge this PR on next before #789.

Halbaroth and others added 3 commits January 2, 2024 12:02
This PR fixes the issue OCamlPro#778. The suggested solution by this PR is as follows:
- Collect user-declared symbols in `D_cnf`;
- As the function `make` of the module `D_cnf` returns a list of
  commands, we add a new command `Decl` in `Frontend` to declare
  symbols;
- The declared symbols are given to the SAT solver through a new
  function `declaration` in `sat_solver_sig`.
- These symbols are collected in vectors and we keep indexes in another
  PR in order to manage properly the push/pop mechanism.
- Finally the declared symbols are given to `Uf` and we create the empty
  model with them.

Another annoying point comes from the current datastructure used in
`ModelMap`. The type of `ModelMap` saves model values in the form of graphs.
A graph is a finite set of constraints but there is no appropriate
representation for the graph without constraints. Let's imagine we have
the input:
```smt2
(set-option :produce-models true)
(set-logic ALL)
(declare-fun (Int) Int)
(check-sat)
(get-model)
```
We expect an output as follows:
```smt2
(
  (define-fun f (_x Int) (@A as Int))
)
```
But `Graph.empty` cannot hold the abstract value `@a`. A naive solution
consists in using an ADT:
```ocaml
type graph =
| Empty of Id.typed
| G of Graph.t
```
But we'll add an extra indirection. Probably the best solution would be
to use a custom datastructure instead of a map to store the constraints.
In this PR, we detect in `ModelMap.add` if the only constraint in the
graph is of the form `something -> abstract value`. In this case we
remove this constraint.

This solution isn't perfect as explained in issue OCamlPro#1018. The biggest
drawback of this solution is the fact we have to declare symbols in the
SAT API, otherwise models returned by `get_model` could be
incomplete.
@Halbaroth
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As I explained during the last dev meeting, we cannot check models of problems involving custom builtin functions like ae.round because there is no mechanism in Dolmen's binary to register custom definitions: Gbury/dolmen#203

But I believe we can do it by adding definitions in the input problems. I think this isn't very important and I'll finish this PR without
supporting this annoying case. In these few cases, the model will be check by get-value anyway ;)

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