diff --git a/src/math/interval/mod_interval.h b/src/math/interval/mod_interval.h
index eaa7e5c98..0e3554a9b 100644
--- a/src/math/interval/mod_interval.h
+++ b/src/math/interval/mod_interval.h
@@ -48,12 +48,23 @@ public:
     mod_interval(Numeral const& l, Numeral const& h): lo(l), hi(h) {}
     static mod_interval free() { return mod_interval(0, 0); }
     static mod_interval empty() { mod_interval i(0, 0); i.emp = true; return i; }
+
     bool is_free() const { return !emp && lo == hi; }
     bool is_empty() const { return emp; }
+    bool is_singleton() const { return !is_empty() && (lo + 1 == hi || (hi == 0 && is_max(lo))); }
+    bool contains(Numeral const& n) const;
+    virtual bool is_max(Numeral const& n) const { return n + 1 == 0; }
+
     void set_free() { lo = hi = 0; emp = false; }
     void set_bounds(Numeral const& l, Numeral const& h) { lo = l; hi = h; }
     void set_empty() { emp = true; }
-    bool contains(Numeral const& n) const;
+
+    void intersect_ule(Numeral const& h);
+    void intersect_uge(Numeral const& l);
+    void intersect_ult(Numeral const& h);
+    void intersect_ugt(Numeral const& l);
+    void intersect_fixed(Numeral const& n);
+    void intersect_diff(Numeral const& n);
     mod_interval operator&(mod_interval const& other) const;
     mod_interval operator+(mod_interval const& other) const;
     mod_interval operator-(mod_interval const& other) const;
diff --git a/src/math/interval/mod_interval_def.h b/src/math/interval/mod_interval_def.h
index 253dd7390..c41054ba3 100644
--- a/src/math/interval/mod_interval_def.h
+++ b/src/math/interval/mod_interval_def.h
@@ -120,3 +120,89 @@ Numeral mod_interval<Numeral>::closest_value(Numeral const& n) const {
         return lo;
     return hi - 1;
 }
+
+// TBD: correctness and completeness for wrap-around semantics needs to be checked/fixed
+
+template<typename Numeral>
+void mod_interval<Numeral>::intersect_ule(Numeral const& h) {
+    if (is_empty())
+        return;
+    if (is_max(h))
+        return;
+    else if (is_free())
+        lo = 0, hi = h + 1;
+    else if (hi > lo && lo > h)
+        set_empty();
+    else if (hi != 0 || h + 1 < hi)
+        hi = h + 1;  
+}
+
+template<typename Numeral>
+void mod_interval<Numeral>::intersect_uge(Numeral const& l) {
+    if (is_empty())
+        return;
+    if (lo < hi && hi <= l)
+        set_empty();
+    else if (is_free())
+        lo = l, hi = 0;
+    else if (lo < hi && lo < l)
+        lo = l;                    
+}
+
+template<typename Numeral>
+void mod_interval<Numeral>::intersect_ult(Numeral const& h) {
+    if (is_empty())
+        return;
+    if (h == 0)
+        set_empty();
+    else if (is_free())
+        lo = 0, hi = h;
+    else if (hi > lo && lo >= h)
+        set_empty();
+    else if (hi > lo && h < hi)
+        hi = h;
+}
+
+template<typename Numeral>
+void mod_interval<Numeral>::intersect_ugt(Numeral const& l) {
+    if (is_empty())
+        return;
+    if (is_max(l))
+        set_empty();
+    else if (is_free())
+        lo = l + 1, hi = 0;
+    else if (lo > l)
+        return;
+    else if (lo < hi && hi <= l)
+        set_empty();
+    else if (lo < hi) 
+        lo = l + 1;
+}
+
+template<typename Numeral>
+void mod_interval<Numeral>::intersect_fixed(Numeral const& a) {
+    if (is_empty())
+        return;
+    if (!contains(a))
+        set_empty();
+    else if (is_max(a))
+        lo = a, hi = 0;
+    else
+        lo = a, hi = a + 1;
+}
+
+template<typename Numeral>
+void mod_interval<Numeral>::intersect_diff(Numeral const& a) {
+    if (!contains(a) || is_empty())
+        return;
+    if (a == lo && a + 1 == hi)
+        set_empty();
+    else if (a == lo && hi == 0 && is_max(a))
+        set_empty();
+    else if (a == lo && !is_max(a))
+        lo = a + 1;
+    else if (a + 1 == hi)
+        hi = a;
+    else if (hi == 0 && is_max(a))
+        hi = a;
+}
diff --git a/src/math/polysat/viable.cpp b/src/math/polysat/viable.cpp
index 584adb17c..40c79975d 100644
--- a/src/math/polysat/viable.cpp
+++ b/src/math/polysat/viable.cpp
@@ -44,139 +44,78 @@ namespace polysat {
         return a + 1 == rational::power_of_two(m_num_bits);
     }
     
-    bool viable_set::is_singleton() const {
-        return !is_empty() && (lo + 1 == hi || (hi == 0 && is_max(lo)));
-    }
-
     void viable_set::intersect_eq(rational const& a, bool is_positive) {
-        if (is_empty())
-            return;
-        if (is_positive) {
-            if (!contains(a))
-                set_empty();
-            else if (is_max(a))
-                lo = a, hi = 0;
-            else
-                lo = a, hi = a + 1;
-        }
-        else {
-            if (!contains(a))
-                return;
-            if (a == lo && a + 1 == hi)
-                set_empty();
-            else if (a == lo && hi == 0 && is_max(a))
-                set_empty();
-            else if (a == lo && !is_max(a))
-                lo = a + 1;
-            else if (a + 1 == hi)
-                hi = a;
-            else if (hi == 0 && is_max(a))
-                hi = a;
-            else 
-                std::cout << "unhandled diseq " << lo << " " << a << " " << hi << "\n";
-        }
+        if (is_positive) 
+            intersect_fixed(a);        
+        else 
+            intersect_diff(a);        
     }
 
     bool viable_set::intersect_eq(rational const& a, rational const& b, bool is_positive) {
-        if (a.is_odd()) {
-            if (b == 0)
-                intersect_eq(b, is_positive);
-            else {
-                rational a_inv;
-                VERIFY(a.mult_inverse(m_num_bits, a_inv));
-                intersect_eq(mod(a_inv * -b, p2()), is_positive);
-            }
-            return true;
+        if (!a.is_odd()) {
+            std::function<bool(rational const&)> eval = [&](rational const& x) {
+                return is_positive == (mod(a * x + b, p2()) == 0);
+            };       
+            return narrow(eval);            
         }
+        if (b == 0)
+            intersect_eq(b, is_positive);
         else {
-            return false;
+            rational a_inv;
+            VERIFY(a.mult_inverse(m_num_bits, a_inv));
+            intersect_eq(mod(a_inv * -b, p2()), is_positive);
         }
+        return true;
     }
 
-    void viable_set::intersect_eq(rational const& a, rational const& b, bool is_positive, unsigned& budget) {
-        std::function<bool(rational const&)> eval = [&](rational const& x) {
-            return is_positive == (mod(a * x + b, p2()) == 0);
-        };       
-        narrow(eval, budget);
-    }
-
-
-    bool viable_set::intersect_ule(rational const& a, rational const& b, rational const& c, rational const& d, bool is_positive) {
+    bool viable_set::intersect_le(rational const& a, rational const& b, rational const& c, rational const& d, bool is_positive) {
         // x <= 0
         if (a.is_odd() && b == 0 && c == 0 && d == 0)
             intersect_eq(b, is_positive);
         else if (a == 1 && b == 0 && c == 0) {
-            // x <= d
+            // x <= d or x > d
             if (is_positive)
-                set_hi(d);
-            // x > d
-            else if (is_max(d))
-                set_empty();
-            else
-                set_lo(d + 1);
+                intersect_ule(d);
+            else 
+                intersect_ugt(d);
         }
         else if (a == 0 && c == 1 && d == 0) {
-            // x >= b
+            // x >= b or x < b
             if (is_positive)
-                set_lo(b);
-            else if (b == 0)
-                set_empty();
+                intersect_uge(b);
             else
-                set_hi(b - 1);
+                intersect_ult(b);
+        }
+        // TBD: can also handle wrap-around semantics (for signed comparison)
+        else {
+            std::function<bool(rational const&)> eval = [&](rational const& x) {
+                return is_positive == mod(a * x + b, p2()) <= mod(c * x + d, p2());
+            };       
+            return narrow(eval);
         }
-        else
-            return false;
-
         return true;
     }
 
-    void viable_set::narrow(std::function<bool(rational const&)>& eval, unsigned& budget) {
-        while (budget > 0 && !eval(lo) && !is_max(lo) && !is_empty()) {
+    rational viable_set::prev(rational const& p) const {
+        if (p > 0)
+            return p - 1;
+        else
+            return rational::power_of_two(m_num_bits) - 1;
+    }
+
+    bool viable_set::narrow(std::function<bool(rational const&)>& eval) {
+        unsigned budget = 10;
+        while (budget > 0 && !is_empty() && !eval(lo)) {
             --budget;
-            lo += 1;
-            set_lo(lo);
+            intersect_diff(lo);
         }
-        while (budget > 0 && hi > 0 && !eval(hi - 1) && !is_empty()) {
+        while (budget > 0 && !is_empty() && !eval(prev(hi))) {
             --budget;
-            hi = hi - 1;
-            set_hi(hi);
+            intersect_diff(prev(hi));
         }
+        return 0 < budget;
     }
 
-    void viable_set::intersect_ule(rational const& a, rational const& b, rational const& c, rational const& d, bool is_positive, unsigned& budget) {
-        std::function<bool(rational const&)> eval = [&](rational const& x) {
-            return is_positive == mod(a * x + b, p2()) <= mod(c * x + d, p2());
-        };       
-        narrow(eval, budget);
-    }
-
-    void viable_set::set_hi(rational const& d) {
-        if (is_max(d))
-            return;
-        else if (is_free())
-            lo = 0, hi = d + 1;
-        else if (lo > d)
-            set_empty();
-        else if (hi != 0 || d + 1 < hi)
-            hi = d + 1;  
-        else if (d + 1 == hi)
-            return;
-        else 
-            std::cout << "set hi " << d << " " << *this << "\n";
-    }
-
-    void viable_set::set_lo(rational const& b) {
-        if (hi != 0 && hi <= b)
-            set_empty();
-        else if (is_free())
-            lo = b, hi = 0;
-        else if (lo < b)
-            lo = b;                    
-        else if (lo == b)
-            return;
-        else 
-            std::cout << "set lo " << b << " " << *this << "\n";
-    }
 #endif
 
     viable::viable(solver& s):
@@ -184,6 +123,18 @@ namespace polysat {
         m_bdd(1000)
     {}
 
+    viable::~viable() {
+#if NEW_VIABLE
+        ptr_vector<cached_constraint> entries;
+        for (auto* e : m_constraint_cache) 
+            entries.push_back(e);
+        m_constraint_cache.reset();
+        for (auto* e : entries)
+            dealloc(e);
+#endif
+    }
+
+
     void viable::push_viable(pvar v) {
         s.m_trail.push_back(trail_instr_t::viable_i);
         m_viable_trail.push_back(std::make_pair(v, m_viable[v]));
@@ -200,15 +151,8 @@ namespace polysat {
     void viable::intersect_eq(rational const& a, pvar v, rational const& b, bool is_positive) {
 #if NEW_VIABLE
         push_viable(v);
-        if (!m_viable[v].intersect_eq(a, b, is_positive)) {
-            IF_VERBOSE(10, verbose_stream() << "could not intersect v" << v << " " << m_viable[v] << "\n");
-            unsigned budget = 10;
-            m_viable[v].intersect_eq(a, b, is_positive, budget);
-            if (budget == 0) {
-                std::cout << "budget used\n";
-                // then narrow the range using BDDs
-            }
-        }
+        if (!m_viable[v].intersect_eq(a, b, is_positive)) 
+            intersect_eq_bdd(v, a, b, is_positive);
         if (m_viable[v].is_empty())
             s.set_conflict(v);
 #else
@@ -239,54 +183,9 @@ namespace polysat {
 
     void viable::intersect_ule(pvar v, rational const& a, rational const& b, rational const& c, rational const& d, bool is_positive) {
 #if NEW_VIABLE
-        // 
-        // TODO This code needs to be partitioned into self-contained pieces.
-        //
         push_viable(v);
-        if (!m_viable[v].intersect_ule(a, b, c, d, is_positive)) {
-            unsigned budget = 10;
-            m_viable[v].intersect_ule(a, b, c, d, is_positive, budget);
-            if (budget == 0) {
-                std::cout << "miss: " << a << " " << b << " " << c << " " << d << " " << is_positive << "\n";
-                unsigned sz = var2bits(v).num_bits();
-                bdd le = m_bdd.mk_true();
-                ineq_entry entry0(sz, a, b, c, d, le);
-                ineq_entry* other = nullptr;
-                if (!m_ineq_cache.find(&entry0, other)) {
-                    std::cout << "ADD-to-cache\n";
-                    bddv const& x = var2bits(v).var();
-                    le = ((a * x) + b) <= ((c * x) + d);
-                    other = alloc(ineq_entry, sz, a, b, c, d, le);
-                    m_ineq_cache.insert(other);
-                }
-                bdd gt = is_positive ? !other->repr : other->repr;
-                other->m_activity++;
-
-                // 
-                // instead of using activity for GC, use the Move-To-Front approach 
-                // see sat/smt/bv_ackerman.h or sat/smt/euf_ackerman.h
-                // where hash table entries use a dll_base.
-                // 
-
-                // le(lo) is false: find min x >= lo, such that le(x) is false, le(x+1) is true
-                // le(hi) is false: find max x =< hi, such that le(x) is false, le(x-1) is true
-                
-                rational bound = m_viable[v].lo;
-                if (var2bits(v).sup(gt, bound)) {
-                    m_viable[v].set_lo(bound);
-                    m_viable[v].set_ne(bound);
-                }
-                bound = m_viable[v].hi;
-                if (bound != 0) {
-                    bound = bound - 1;
-                    if (var2bits(v).inf(gt, bound)) {
-                        std::cout << "TODO: new upper bound " << bound << "\n";
-                    }
-                }
-
-            }
-
-        }
+        if (!m_viable[v].intersect_le(a, b, c, d, is_positive)) 
+            intersect_ule_bdd(v, a, b, c, d, is_positive);
         if (m_viable[v].is_empty())
             s.set_conflict(v);
 #else
@@ -305,6 +204,77 @@ namespace polysat {
 #endif
     }
 
+#if NEW_VIABLE
+
+    viable::cached_constraint& viable::cache_constraint(pvar v, cached_constraint& entry0, std::function<bdd(void)>& mk_constraint) {        
+        cached_constraint* other = nullptr;
+        if (!m_constraint_cache.find(&entry0, other)) {
+            gc_cached_constraints();
+            other = alloc(cached_constraint, entry0);
+            other->repr = mk_constraint();
+            m_constraint_cache.insert(other);
+        }
+        other->m_activity++;
+        return *other;
+    }
+
+    void viable::gc_cached_constraints() {
+        // 
+        // TODO: instead of using activity for GC, use the Move-To-Front approach 
+        // see sat/smt/bv_ackerman.h or sat/smt/euf_ackerman.h
+        // where hash table entries use a dll_base.
+        // 
+        unsigned max_entries = 10000;
+        if (m_constraint_cache.size() > max_entries) {
+            ptr_vector<cached_constraint> entries;
+            for (auto* e : m_constraint_cache) 
+                entries.push_back(e);
+            std::stable_sort(entries.begin(), entries.end(), [&](cached_constraint* a, cached_constraint* b) { return a->m_activity < b->m_activity; });
+            for (unsigned i = 0; i < max_entries/2; ++i) {
+                m_constraint_cache.remove(entries[i]);
+                dealloc(entries[i]);
+            }
+        }
+    }
+
+    void viable::narrow(pvar v, bdd const& is_false) {        
+        rational bound = m_viable[v].lo;
+        if (var2bits(v).sup(is_false, bound)) 
+            m_viable[v].intersect_ugt(bound);
+        bound = m_viable[v].prev(m_viable[v].hi);
+        if (var2bits(v).inf(is_false, bound)) 
+            m_viable[v].intersect_ult(bound);
+    }
+
+    void viable::intersect_ule_bdd(pvar v, rational const& a, rational const& b, rational const& c, rational const& d, bool is_positive) {
+        unsigned sz = var2bits(v).num_bits();
+        std::function<bdd(void)> le = [&]() {
+            bddv const& x = var2bits(v).var();
+            return ((a * x) + b) <= ((c * x) + d);
+        };
+        cached_constraint entry0(sz, a, b, c, d, m_bdd.mk_true());
+        cached_constraint& entry = cache_constraint(v, entry0, le);
+
+        // le(lo) is false: find min x >= lo, such that le(x) is false, le(x+1) is true
+        // le(hi) is false: find max x =< hi, such that le(x) is false, le(x-1) is true
+        bdd gt = is_positive ? !entry.repr : entry.repr;
+        narrow(v, gt);        
+    }
+
+    void viable::intersect_eq_bdd(pvar v, rational const& a, rational const& b, bool is_positive) {
+        unsigned sz = var2bits(v).num_bits();
+        std::function<bdd(void)> eq = [&]() {
+            bddv const& x = var2bits(v).var();
+            return ((a * x) + b) == rational(0);
+        };
+        cached_constraint entry0(sz, a, b, m_bdd.mk_true());
+        cached_constraint& entry = cache_constraint(v, entry0, eq);
+
+        bdd ne = is_positive ? !entry.repr : entry.repr;
+        narrow(v, ne);        
+    }
+#endif
+
     bool viable::has_viable(pvar v) {
 #if NEW_VIABLE
         return !m_viable[v].is_empty();
@@ -325,7 +295,7 @@ namespace polysat {
 #if NEW_VIABLE
         push_viable(v);
         IF_VERBOSE(10, verbose_stream() << " v" << v << " != " << val << "\n");
-        m_viable[v].set_ne(val);
+        m_viable[v].intersect_diff(val);
         if (m_viable[v].is_empty())
             s.set_conflict(v);
 #else
diff --git a/src/math/polysat/viable.h b/src/math/polysat/viable.h
index 7c17c8510..ddec88412 100644
--- a/src/math/polysat/viable.h
+++ b/src/math/polysat/viable.h
@@ -14,12 +14,11 @@ Author:
 Notes:
 
     NEW_VIABLE uses cheaper book-keeping, but is partial.
-    The implementation of NEW_VIABLE is atm incomplete and ad-hoc.
     
 --*/
 #pragma once
 
-#define NEW_VIABLE 0
+#define NEW_VIABLE 1
 
 #include <limits>
 
@@ -44,52 +43,57 @@ namespace polysat {
     class viable_set : public mod_interval<rational> {
         unsigned m_num_bits;
         rational p2() const { return rational::power_of_two(m_num_bits); }
-        bool is_max(rational const& a) const;
+        bool is_max(rational const& a) const override;
         void intersect_eq(rational const& a, bool is_positive);
-        void narrow(std::function<bool(rational const&)>& eval, unsigned& budget);
+        bool narrow(std::function<bool(rational const&)>& eval);
     public:
         viable_set(unsigned num_bits): m_num_bits(num_bits) {}
-        bool is_singleton() const; 
         dd::find_t find_hint(rational const& c, rational& val) const;
-        void set_ne(rational const& a) { intersect_eq(a, false); }
-        void set_lo(rational const& lo);
-        void set_hi(rational const& hi);
         bool intersect_eq(rational const& a, rational const& b, bool is_positive);
-        void intersect_eq(rational const& a, rational const& b, bool is_positive, unsigned& budget);
-        bool intersect_ule(rational const& a, rational const& b, rational const& c, rational const& d, bool is_positive);
-        void intersect_ule(rational const& a, rational const& b, rational const& c, rational const& d, bool is_positive, unsigned& budget);
+        bool intersect_le(rational const& a, rational const& b, rational const& c, rational const& d, bool is_positive);
+        rational prev(rational const& p) const;
     };
 #endif
 
     class viable {
-        solver& s;
         typedef dd::bdd bdd;
         typedef dd::fdd fdd;
+        solver&                      s;
         dd::bdd_manager              m_bdd;
         scoped_ptr_vector<dd::fdd>   m_bits;
 #if NEW_VIABLE
-        struct ineq_entry {
+        struct cached_constraint {
+            enum op_code { is_ule, is_eq };
+            op_code  m_op;
             unsigned m_num_bits;
             rational a, b, c, d;
             bdd      repr;
             unsigned m_activity = 0;
-            ineq_entry(unsigned n, rational const& a, rational const& b, rational const& c, rational const& d, bdd& f) :
-                m_num_bits(n), a(a), b(b), c(c), d(d), repr(f) {}
+            cached_constraint(unsigned n, rational const& a, rational const& b, rational const& c, rational const& d, bdd& f) :
+                m_op(op_code::is_ule), m_num_bits(n), a(a), b(b), c(c), d(d), repr(f) {}
+            cached_constraint(unsigned n, rational const& a, rational const& b, bdd& f) :
+                m_op(op_code::is_eq), m_num_bits(n), a(a), b(b), repr(f) {}
 
             struct hash {
-                unsigned operator()(ineq_entry const* e) const {
-                    return mk_mix(e->a.hash(), e->b.hash(), mk_mix(e->c.hash(), e->d.hash(), e->m_num_bits));
+                unsigned operator()(cached_constraint const* e) const {
+                    return mk_mix(e->a.hash(), e->b.hash(), mk_mix(e->c.hash(), e->d.hash(), e->m_num_bits)) + e->m_op;
                 }
             };
             struct eq {
-                bool operator()(ineq_entry const* x, ineq_entry const* y) const {
-                    return x->a == y->a && x->b == y->b && x->c == y->c && x->d == y->d && x->m_num_bits == y->m_num_bits;
+                bool operator()(cached_constraint const* x, cached_constraint const* y) const {
+                    return x->m_op == y->m_op && x->a == y->a && x->b == y->b && x->c == y->c && x->d == y->d && x->m_num_bits == y->m_num_bits;
                 }
             };
         };
         vector<viable_set>                                       m_viable; 
         vector<std::pair<pvar, viable_set>>                      m_viable_trail;
-        hashtable<ineq_entry*, ineq_entry::hash, ineq_entry::eq> m_ineq_cache;
+        hashtable<cached_constraint*, cached_constraint::hash, cached_constraint::eq> m_constraint_cache;
+
+        void intersect_ule_bdd(pvar v, rational const& a, rational const& b, rational const& c, rational const& d, bool is_positive);
+        void intersect_eq_bdd(pvar v, rational const& a, rational const& b, bool is_positive);
+        cached_constraint& cache_constraint(pvar v, cached_constraint& entry0, std::function<bdd(void)>& mk_constraint);
+        void gc_cached_constraints();
+        void narrow(pvar v, bdd const& is_false);
 #else
 
         
@@ -110,6 +114,8 @@ namespace polysat {
     public:
         viable(solver& s);
 
+        ~viable();
+
         void push(unsigned num_bits) { 
 #if NEW_VIABLE
             m_viable.push_back(viable_set(num_bits));