diff --git a/src/muz/rel/doc.cpp b/src/muz/rel/doc.cpp
index 4f950973c..7cad5ffe8 100644
--- a/src/muz/rel/doc.cpp
+++ b/src/muz/rel/doc.cpp
@@ -119,6 +119,12 @@ bool doc_manager::set_and(doc& dst, doc const& src)  {
     }
     return (src.neg().is_empty() || fold_neg(dst));
 }
+bool doc_manager::set_and(doc& dst, tbv const& src)  {
+    // (A \ B) & C  = (A & C) \ B
+    if (!m.set_and(dst.pos(), src)) return false;
+    dst.neg().intersect(m, src);
+    return true;
+}
 
 bool doc_manager::well_formed(doc const& d) const {
     if (!m.is_well_formed(d.pos())) return false;
@@ -404,23 +410,21 @@ void doc_manager::complement(doc const& src, ptr_vector<doc>& result) {
 // (A & !A1 & & !B) |  (A & B1 & !A1)
 // A \ {A1 u B} u (A & B1) \ {A1}
 void doc_manager::subtract(doc const& A, doc const& B, ptr_vector<doc>& result) {
-    doc_ref r(*this), r2(*this);
+    doc_ref r(*this);
     tbv_ref t(m);
     r = allocate(A);
     t = m.allocate(B.pos());
     if (m.set_and(*t, A.pos()) && r->neg().insert(m, t.detach())) {
         result.push_back(r.detach());
-        r = allocate(A);
     }
     else {
         result.push_back(allocate(A));
     }
     for (unsigned i = 0; i < B.neg().size(); ++i) {
-        r2 = allocate(B.neg()[i]);
-        if (set_and(*r, *r2)) {
+        r = allocate(A);
+        if (set_and(*r, B.neg()[i])) {
             result.push_back(r.detach());
         }
-        r = allocate(A);
     }
 }
 bool doc_manager::equals(doc const& a, doc const& b) const {
diff --git a/src/muz/rel/doc.h b/src/muz/rel/doc.h
index bf2cdc229..3c2b9091d 100644
--- a/src/muz/rel/doc.h
+++ b/src/muz/rel/doc.h
@@ -59,6 +59,7 @@ public:
     bool is_full(doc const& src) const;
     bool is_empty(doc const& src);
     bool set_and(doc& dst, doc const& src);
+    bool set_and(doc& dst, tbv const& src);
     bool fold_neg(doc& dst);
     bool intersect(doc const& A, doc const& B, doc& result);
     void complement(doc const& src, ptr_vector<doc>& result);
diff --git a/src/muz/rel/udoc_relation.cpp b/src/muz/rel/udoc_relation.cpp
index 7ef3df3c2..1d0da90dd 100644
--- a/src/muz/rel/udoc_relation.cpp
+++ b/src/muz/rel/udoc_relation.cpp
@@ -753,16 +753,17 @@ namespace datalog {
         expr* e1, *e2;
         if (result.is_empty()) {
         }
+        else if (m.is_true(g)) {
+        }
+        else if (m.is_false(g)) {
+            result.reset(dm);
+        }
         else if (m.is_and(g)) {
             for (unsigned i = 0; !result.is_empty() && i < to_app(g)->get_num_args(); ++i) {
                 apply_guard(to_app(g)->get_arg(i), result, equalities, discard_cols);
             }
         }
         else if (m.is_not(g, e1)) {
-            // REVIEW: (not (= x y)) should not cause 
-            // the equivalence class to collapse.
-            // It seems the current organization with fix_eq_bits
-            // will merge the equivalence class as a side-effect.
             udoc sub;
             sub.push_back(dm.allocateX());
             apply_guard(e1, sub, equalities, discard_cols);
@@ -789,11 +790,6 @@ namespace datalog {
                   result.display(dm, tout << "result:") << "\n";);
             sub.reset(dm);
         }
-        else if (m.is_true(g)) {
-        }
-        else if (m.is_false(g)) {
-            result.reset(dm);
-        }
         else if (is_var(g)) {
             SASSERT(m.is_bool(g));
             unsigned v = to_var(g)->get_idx();
@@ -885,10 +881,8 @@ namespace datalog {
             SASSERT(u.well_formed(dm));
             u.intersect(dm, m_udoc);
             SASSERT(u.well_formed(dm));
-            if (!m.is_true(m_condition) && !u.is_empty()) {
-                t.apply_guard(m_condition, u, m_equalities, m_empty_bv);
-                SASSERT(u.well_formed(dm));
-            }
+            t.apply_guard(m_condition, u, m_equalities, m_empty_bv);
+            SASSERT(u.well_formed(dm));            
             u.simplify(dm);
             SASSERT(u.well_formed(dm));
             TRACE("doc", tout << "final size: " << t.get_size_estimate_rows() << '\n';);
@@ -1041,10 +1035,8 @@ namespace datalog {
             SASSERT(u2.well_formed(dm));
             u2.merge(dm, m_roots, m_equalities, m_col_list);
             SASSERT(u2.well_formed(dm)); 
-            if (!m.is_true(m_condition) && !u2.is_empty()) {
-                t.apply_guard(m_condition, u2, m_equalities, m_col_list);
-                SASSERT(u2.well_formed(dm));
-            }
+            t.apply_guard(m_condition, u2, m_equalities, m_col_list);
+            SASSERT(u2.well_formed(dm));            
             udoc_relation* r = get(t.get_plugin().mk_empty(get_result_signature()));
             doc_manager& dm2 = r->get_dm();
             for (unsigned i = 0; i < u2.size(); ++i) {