wip - updated version of elim_uncstr_tactic

- remove reduce_invertible. It is subsumed by reduce_uncstr(2)
- introduce a simplifier for reduce_unconstrained. It uses reference counting to deal with inefficiency bug of legacy reduce_uncstr. It decomposes theory plugins into expr_inverter.

reduce_invertible is a tactic used in most built-in scenarios. It is useful for removing subterms that can be eliminated using "cheap" quantifier elimination. Specifically variables that occur only once can be removed in many cases by computing an expression that represents the effect computing a value for the eliminated occurrence.

The theory plugins for variable elimination are very partial and should be augmented by extensions, esp. for the case of bit-vectors where the invertibility conditions are thoroughly documented by Niemetz and Preiner.

src/ast/converters/expr_inverter.h

@@ -0,0 +1,57 @@
+/*++
+Copyright (c) 2017 Microsoft Corporation
+
+Module Name:
+
+    expr_inverter.h
+
+Abstract:
+
+    inverter interface and instance
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2022-10-11
+
+
+--*/
+#pragma once
+
+#include "ast/converters/generic_model_converter.h"
+
+class iexpr_inverter {
+protected:
+    ast_manager&                m;
+    std::function<bool(expr*)>  m_is_var;
+    generic_model_converter_ref m_mc;
+
+    bool uncnstr(expr* e) const { return m_is_var(e); }
+    bool uncnstr(unsigned num, expr * const * args) const;
+    void mk_fresh_uncnstr_var_for(sort* s, expr_ref& v);
+    void mk_fresh_uncnstr_var_for(func_decl* f, expr_ref& v) { mk_fresh_uncnstr_var_for(f->get_range(), v); }
+    void add_def(expr * v, expr * def);
+    void add_defs(unsigned num, expr * const * args, expr * u, expr * identity);
+
+public:
+    iexpr_inverter(ast_manager& m): m(m) {}
+    virtual ~iexpr_inverter() {}
+    virtual void set_is_var(std::function<bool(expr*)>& is_var) { m_is_var = is_var; }
+    virtual void set_model_converter(generic_model_converter* mc) { m_mc = mc; }
+    
+    virtual bool operator()(func_decl* f, unsigned n, expr* const* args, expr_ref& new_expr, expr_ref& side_cond) = 0;
+    virtual bool mk_diff(expr* t, expr_ref& r) = 0;
+};
+
+class expr_inverter : public iexpr_inverter {
+    ptr_vector<iexpr_inverter> m_inverters;
+
+    bool is_converted(func_decl* f, unsigned num, expr* const* args);
+
+public:
+    expr_inverter(ast_manager& m);
+    ~expr_inverter() override;
+    bool operator()(func_decl* f, unsigned n, expr* const* args, expr_ref& new_expr, expr_ref& side_cond) override;
+    bool mk_diff(expr* t, expr_ref& r) override;
+    void set_is_var(std::function<bool(expr*)>& is_var) override; 
+    void set_model_converter(generic_model_converter* mc) override;
+};