refactor occurrence utility for common use (to be used in ctx_simplifier) per Nuno's suggestion

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

src/tactic/core/collect_occs.h

@@ -0,0 +1,38 @@
+/*++
+Copyright (c) 2011 Microsoft Corporation
+
+Module Name:
+
+    collect_cccs.h
+
+Abstract:
+
+    Collect variables by occurrences.
+
+Author:
+
+    Leonardo (leonardo) 2011-10-22
+
+Notes:
+
+--*/
+#ifndef COLLECT_OCCS_H_
+#define COLLECT_OCCS_H_
+
+class collect_occs {
+    expr_fast_mark1  m_visited;
+    expr_fast_mark2  m_more_than_once;
+    typedef std::pair<expr *, unsigned> frame;
+    svector<frame>   m_stack;
+    ptr_vector<app>  m_vars;
+    
+    bool visit(expr * t);
+    void process(expr * t);
+    
+public:
+    
+    void operator()(goal const & g, obj_hashtable<expr> & r);
+
+};
+
+#endif