add smt lookahead

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/smt/smt_lookahead.h

@@ -0,0 +1,40 @@
+/*++
+Copyright (c) 2006 Microsoft Corporation
+
+Module Name:
+
+    smt_lookahead.h
+
+Abstract:
+
+    Lookahead solver for SMT
+
+Author:
+
+    nbjorner 2019-05-27.
+
+Revision History:
+
+--*/
+
+#pragma once
+
+#include "ast/ast.h"
+
+namespace smt {
+    class context;
+
+    class lookahead {
+        context&     ctx;
+        ast_manager& m;
+
+        struct compare;
+
+        double get_score();
+
+    public:
+        lookahead(context& ctx);
+
+        expr_ref choose();
+    };
+}