merge changes from Z3Prover

Signed-off-by: Lev Nachmanson <levnach@hotmail.com>

src/math/grobner/pdd_grobner.cpp

@@ -698,6 +698,7 @@ namespace dd {
                 }
             }
             if (eq) {
+
                 pop_equation(eq);
                 m_watch[eq->poly().var()].erase(eq);
                 return eq;

src/math/grobner/pdd_solver.h

@@ -130,6 +130,7 @@ public:
 private:
     bool step();
     equation* pick_next();
+    equation* pick_linear();
     bool canceled();
     bool done();
     void superpose(equation const& eq1, equation const& eq2);