fix build, add ZDD reference

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

src/math/dd/dd_pdd.h

@@ -9,6 +9,9 @@ Abstract:
 
     Poly DD package
 
+    It is a mild variant of ZDDs. 
+    In PDDs arithmetic is either standard or using mod 2 (over GF2).
+
     Non-leaf nodes are of the form x*hi + lo
     where 
     - maxdegree(x, lo) = 0, 

src/math/grobner/pdd_grobner.cpp

@@ -425,7 +425,7 @@ namespace dd {
         // they watch the top most variable in poly
         i = 0;
         for (auto const& w : m_watch) {
-            for (equation const* e : w) {
+            for (equation* e : w) {
                 VERIFY(!e->poly().is_val());
                 VERIFY(e->poly().var() == i);
                 VERIFY(!e->is_processed());