This update changes the handling of mod and adds support for nested div terms.
Simple use cases that are handled using small results are given below.
```
(declare-const x Int)
(declare-const y Int)
(declare-const z Int)
(assert (exists ((x Int)) (and (<= y (* 10 x)) (<= (* 10 x) z))))
(apply qe2)
(reset)
(declare-const y Int)
(assert (exists ((x Int)) (and (> x 0) (= (div x 41) y))))
(apply qe2)
(reset)
(declare-const y Int)
(assert (exists ((x Int)) (= (mod x 41) y)))
(apply qe2)
(reset)
```
The main idea is to introduce definition rows for mod/div terms.
Elimination of variables under mod/div is defined by rewriting the variable to multiples of the mod/divisior and remainder.
The functionality is disabled in this push.
\brief convert p == 0 into a solved form v == r, such that
v has bounds [lo, oo) iff r has bounds [lo', oo)
v has bounds (oo,hi] iff r has bounds (oo,hi']
The solved form allows the Grobner solver identify more bounds conflicts.
A bad leading term can miss bounds conflicts.
For example for x + y + z == 0 where x, y : [0, oo) and z : (oo,0]
we prefer to solve z == -x - y instead of x == -z - y
because the solution -z - y has neither an upper, nor a lower bound.
The Grobner solver is augmented with a notion of a substitution that is applied before the solver is run.
For Grobner we want to preserve directions of intervals for finding sign conflicts. This means that it makes sense to have external control over linear solutions.
this update integrates inferences to smt.arith.solver=6 related to grobner basis computation and handling of div/mod axioms to reconcile performance with smt.arith.solver=2.
The default of smt.arth.nl.grobner_subs_fixed is changed to 1 to make comparison with solver=2 more direct.
The selection of cluster equalities for solver=6 was reconciled with how it is done for solver=2.
