add EUF plugin framework.

plugin setting allows adding equality saturation within the E-graph propagation without involving externalizing theory solver dispatch. It makes equality saturation independent of SAT integration.
Add a special relation operator to support ad-hoc AC symbols.

src/ast/euf/euf_specrel_plugin.cpp

@@ -0,0 +1,71 @@
+/*++
+Copyright (c) 2023 Microsoft Corporation
+
+Module Name:
+
+    euf_specrel_plugin.cpp
+
+Abstract:
+
+    plugin structure for specrel
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2023-11-11
+
+--*/
+
+#include "ast/euf/euf_specrel_plugin.h"
+#include "ast/euf/euf_egraph.h"
+#include <algorithm>
+
+namespace euf {
+
+    specrel_plugin::specrel_plugin(egraph& g) :
+        plugin(g),
+        sp(g.get_manager()) {
+    }    
+
+    void specrel_plugin::register_node(enode* n) {
+        func_decl* f = n->get_decl();
+        if (!f)
+            return;
+        if (!sp.is_ac(f))
+            return;
+        ac_plugin* p = nullptr;
+        if (!m_decl2plugin.find(f, p)) {
+            p = alloc(ac_plugin, g, f);
+            m_decl2plugin.insert(f, p);
+            m_plugins.push_back(p);
+            std::function<void(void)> undo_op = [&]() { m_undo.push_back(p); };
+            p->set_undo(undo_op);
+        }
+    }
+
+    void specrel_plugin::merge_eh(enode* n1, enode* n2) {
+        for (auto * p : m_plugins)
+            p->merge_eh(n1, n2);
+    }
+
+    void specrel_plugin::diseq_eh(enode* eq) {
+        for (auto* p : m_plugins)
+            p->diseq_eh(eq);
+    }
+
+    void specrel_plugin::propagate() {
+        for (auto * p : m_plugins)
+            p->propagate();        
+    }
+
+    void specrel_plugin::undo() {
+        auto p = m_undo.back();
+        m_undo.pop_back();
+        p->undo();
+    }
+        
+    std::ostream& specrel_plugin::display(std::ostream& out) const {
+        for (auto * p : m_plugins)
+            p->display(out);
+        return out;
+    }
+}