diff --git a/src/muz/spacer/spacer_context.cpp b/src/muz/spacer/spacer_context.cpp
index 48550df12..ff63ae2ef 100644
--- a/src/muz/spacer/spacer_context.cpp
+++ b/src/muz/spacer/spacer_context.cpp
@@ -66,8 +66,8 @@ pob::pob (pob* parent, pred_transformer& pt,
     m_binding(m_pt.get_ast_manager()),
     m_new_post (m_pt.get_ast_manager ()),
     m_level (level), m_depth (depth),
-    m_open (true), m_use_farkas (true), m_weakness(0),
-    m_blocked_lvl(0) {
+    m_open (true), m_use_farkas (true), m_in_queue(false),
+    m_weakness(0), m_blocked_lvl(0) {
     if (add_to_parent && m_parent) {
         m_parent->add_child(*this);
     }
@@ -79,6 +79,7 @@ void pob::set_post(expr* post) {
 }
 
 void pob::set_post(expr* post, app_ref_vector const &binding) {
+    SASSERT(!is_in_queue());
     normalize(post, m_post,
               m_pt.get_context().simplify_pob(),
               m_pt.get_context().use_euf_gen());
@@ -88,6 +89,7 @@ void pob::set_post(expr* post, app_ref_vector const &binding) {
 }
 
 void pob::inherit(pob const &p) {
+    SASSERT(!is_in_queue());
     SASSERT(m_parent == p.m_parent);
     SASSERT(&m_pt == &p.m_pt);
     SASSERT(m_post == p.m_post);
@@ -105,17 +107,10 @@ void pob::inherit(pob const &p) {
     m_derivation = nullptr;
 }
 
-void pob::clean () {
-    if (m_new_post) {
-        m_post = m_new_post;
-        m_new_post.reset();
-    }
-}
-
 void pob::close () {
     if (!m_open) { return; }
 
-    reset ();
+    m_derivation = nullptr;
     m_open = false;
     for (unsigned i = 0, sz = m_kids.size (); i < sz; ++i)
     { m_kids [i]->close(); }
@@ -148,6 +143,13 @@ pob* pob_queue::top ()
     return m_data.top ();
 }
 
+void pob_queue::pop() {
+    pob *p = m_data.top();
+    p->set_in_queue(false);
+    m_data.pop();
+}
+
+
 void pob_queue::set_root(pob& root)
 {
     m_root = &root;
@@ -156,19 +158,28 @@ void pob_queue::set_root(pob& root)
     reset();
 }
 
-pob_queue::~pob_queue() {}
-
 void pob_queue::reset()
 {
-    while (!m_data.empty()) { m_data.pop(); }
-    if (m_root) { m_data.push(m_root.get()); }
+    while (!m_data.empty()) {
+        pob *p = m_data.top();
+        m_data.pop();
+        p->set_in_queue(false);
+    }
+    if (m_root) {
+        SASSERT(!m_root->is_in_queue());
+        m_root->set_in_queue(true);
+        m_data.push(m_root.get());
+    }
 }
 
 void pob_queue::push(pob &n) {
-    TRACE("pob_queue",
-          tout << "pob_queue::push(" << n.post()->get_id() << ")\n";);
-    m_data.push (&n);
-    n.get_context().new_pob_eh(&n);
+    if (!n.is_in_queue()) {
+        TRACE("pob_queue",
+              tout << "pob_queue::push(" << n.post()->get_id() << ")\n";);
+        n.set_in_queue(true);
+        m_data.push (&n);
+        n.get_context().new_pob_eh(&n);
+    }
 }
 
 // ----------------
@@ -2135,7 +2146,7 @@ pob* pred_transformer::pob_manager::mk_pob(pob *parent,
         auto &buf = m_pobs[p.post()];
         for (unsigned i = 0, sz = buf.size(); i < sz; ++i) {
             pob *f = buf.get(i);
-            if (f->parent() == parent) {
+            if (f->parent() == parent && !f->is_in_queue()) {
                 f->inherit(p);
                 return f;
             }
@@ -3051,27 +3062,19 @@ bool context::check_reachability ()
         }
 
         SASSERT (m_pob_queue.top ());
-        // -- remove all closed nodes and updated all dirty nodes
+        // -- remove all closed nodes
         // -- this is necessary because there is no easy way to
         // -- remove nodes from the priority queue.
-        while (m_pob_queue.top ()->is_closed () ||
-               m_pob_queue.top()->is_dirty()) {
-            pob_ref n = m_pob_queue.top ();
-            m_pob_queue.pop ();
-            if (n->is_closed()) {
-                IF_VERBOSE (1,
-                            verbose_stream () << "Deleting closed node: "
-                            << n->pt ().head ()->get_name ()
-                            << "(" << n->level () << ", " << n->depth () << ")"
-                            << " " << n->post ()->get_id () << "\n";);
-                if (m_pob_queue.is_root(*n)) { return true; }
-                SASSERT (m_pob_queue.top ());
-            } else if (n->is_dirty()) {
-                n->clean ();
-                // -- the node n might not be at the top after it is cleaned
-                m_pob_queue.push (*n);
-            } else
-            { UNREACHABLE(); }
+        while (m_pob_queue.top ()->is_closed ()) {
+            pob_ref n = m_pob_queue.top();
+            m_pob_queue.pop();
+            IF_VERBOSE (1,
+                        verbose_stream () << "Deleting closed node: "
+                        << n->pt ().head ()->get_name ()
+                        << "(" << n->level () << ", " << n->depth () << ")"
+                        << " " << n->post ()->get_id () << "\n";);
+            if (m_pob_queue.is_root(*n)) {return true;}
+            SASSERT (m_pob_queue.top ());
         }
 
         SASSERT (m_pob_queue.top ());
@@ -3165,6 +3168,7 @@ bool context::is_reachable(pob &n)
     unsigned num_reuse_reach = 0;
 
     unsigned saved = n.level ();
+    // TBD: don't expose private field
     n.m_level = infty_level ();
     lbool res = n.pt().is_reachable(n, nullptr, &mdl,
                                     uses_level, is_concrete, r,
@@ -3409,10 +3413,11 @@ lbool context::expand_pob(pob& n, pob_ref_buffer &out)
 
         // Optionally update the node to be the negation of the lemma
         if (v && m_use_lemma_as_pob) {
-            n.new_post (mk_and(lemma->get_cube()));
-            n.set_farkas_generalizer (false);
-            // XXX hack while refactoring is in progress
-            n.clean();
+            NOT_IMPLEMENTED_YET();
+            // n.new_post (mk_and(lemma->get_cube()));
+            // n.set_farkas_generalizer (false);
+            // // XXX hack while refactoring is in progress
+            // n.clean();
         }
 
         // schedule the node to be placed back in the queue
diff --git a/src/muz/spacer/spacer_context.h b/src/muz/spacer/spacer_context.h
index 480b8ecf5..f2242f0ad 100644
--- a/src/muz/spacer/spacer_context.h
+++ b/src/muz/spacer/spacer_context.h
@@ -562,6 +562,7 @@ public:
  * A proof obligation.
  */
 class pob {
+    // TBD: remove this
     friend class context;
     unsigned m_ref_count;
     /// parent node
@@ -576,14 +577,15 @@ class pob {
     /// new post to be swapped in for m_post
     expr_ref                m_new_post;
     /// level at which to decide the post
-    unsigned                m_level;
-
-    unsigned                m_depth;
+    unsigned                m_level:16;
+    unsigned                m_depth:16;
 
     /// whether a concrete answer to the post is found
-    bool                    m_open;
+    unsigned                m_open:1;
     /// whether to use farkas generalizer to construct a lemma blocking this node
-    bool                    m_use_farkas;
+    unsigned                m_use_farkas:1;
+    /// true if this pob is in pob_queue
+    unsigned                m_in_queue:1;
 
     unsigned                m_weakness;
     /// derivation representing the position of this node in the parent's rule
@@ -598,17 +600,25 @@ class pob {
     // depth -> watch
     std::map<unsigned, stopwatch> m_expand_watches;
     unsigned m_blocked_lvl;
+
+    void set_post(expr *post);
 public:
     pob (pob* parent, pred_transformer& pt,
          unsigned level, unsigned depth=0, bool add_to_parent=true);
 
     ~pob() {if (m_parent) { m_parent->erase_child(*this); }}
 
+    // TBD: move into constructor and make private
+    void set_post(expr *post, app_ref_vector const &binding);
+
     unsigned weakness() {return m_weakness;}
     void bump_weakness() {m_weakness++;}
     void reset_weakness() {m_weakness=0;}
 
-    void inc_level () {m_level++; m_depth++;reset_weakness();}
+    void inc_level () {
+        SASSERT(!is_in_queue());
+        m_level++; m_depth++;reset_weakness();
+    }
 
     void inherit(pob const &p);
     void set_derivation (derivation *d) {m_derivation = d;}
@@ -628,32 +638,25 @@ public:
     unsigned level () const { return m_level; }
     unsigned depth () const {return m_depth;}
     unsigned width () const {return m_kids.size();}
-    unsigned blocked_at(unsigned lvl=0){return (m_blocked_lvl = std::max(lvl, m_blocked_lvl)); }
+    unsigned blocked_at(unsigned lvl=0){
+        return (m_blocked_lvl = std::max(lvl, m_blocked_lvl));
+    }
 
+    bool is_in_queue() const {return m_in_queue;}
+    void set_in_queue(bool v) {m_in_queue = v;}
     bool use_farkas_generalizer () const {return m_use_farkas;}
     void set_farkas_generalizer (bool v) {m_use_farkas = v;}
 
     expr* post() const { return m_post.get (); }
-    void set_post(expr *post);
-    void set_post(expr *post, app_ref_vector const &binding);
-
-    /// indicate that a new post should be set for the node
-    void new_post(expr *post) {if (post != m_post) {m_new_post = post;}}
-    /// true if the node needs to be updated outside of the priority queue
-    bool is_dirty () {return m_new_post;}
-    /// clean a dirty node
-    void clean();
-
-    void reset () {clean (); m_derivation = nullptr; m_open = true;}
 
     bool is_closed () const { return !m_open; }
     void close();
 
-    const ptr_vector<pob> &children() {return m_kids;}
+    const ptr_vector<pob> &children() const {return m_kids;}
     void add_child (pob &v) {m_kids.push_back (&v);}
     void erase_child (pob &v) {m_kids.erase (&v);}
 
-    const ptr_vector<lemma> &lemmas() {return m_lemmas;}
+    const ptr_vector<lemma> &lemmas() const {return m_lemmas;}
     void add_lemma(lemma* new_lemma) {m_lemmas.push_back(new_lemma);}
 
     bool is_ground () const { return m_binding.empty (); }
@@ -666,8 +669,14 @@ public:
      */
     void get_skolems(app_ref_vector& v);
 
-    void on_expand() { m_expand_watches[m_depth].start(); if (m_parent.get()){m_parent.get()->on_expand();} }
-    void off_expand() { m_expand_watches[m_depth].stop(); if (m_parent.get()){m_parent.get()->off_expand();} };
+    void on_expand() {
+        m_expand_watches[m_depth].start();
+        if (m_parent.get()){m_parent.get()->on_expand();}
+    }
+    void off_expand() {
+        m_expand_watches[m_depth].stop();
+        if (m_parent.get()){m_parent.get()->off_expand();}
+    }
     double get_expand_time(unsigned depth) { return m_expand_watches[depth].get_seconds();}
 
     void inc_ref () {++m_ref_count;}
@@ -782,18 +791,22 @@ class pob_queue {
 
 public:
     pob_queue(): m_root(nullptr), m_max_level(0), m_min_depth(0) {}
-    ~pob_queue();
+    ~pob_queue() {}
 
     void reset();
     pob* top();
-    void pop() {m_data.pop ();}
+    void pop();
     void push (pob &n);
 
     void inc_level () {
         SASSERT (!m_data.empty () || m_root);
         m_max_level++;
         m_min_depth++;
-        if (m_root && m_data.empty()) { m_data.push(m_root.get()); }
+        if (m_root && m_data.empty()) {
+            SASSERT(!m_root->is_in_queue());
+            m_root->set_in_queue(true);
+            m_data.push(m_root.get());
+        }
     }
 
     pob& get_root() const {return *m_root.get ();}