diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index b732670a6..258bbee92 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -2278,6 +2278,13 @@ expr_ref seq_rewriter::derivative(expr* elem, expr* r) {
         result = array.mk_select(2, args);
         result = kleene_predicate(result, seq_sort);
     }
+    else if (m().is_ite(r, p, r1, r2)) {
+        dr1 = derivative(elem, r1);
+        dr2 = derivative(elem, r2);
+        if (dr1 && dr2) {
+            result = m().mk_ite(p, dr1, dr2);
+        }
+    }
     return result;
 }
 
diff --git a/src/ast/seq_decl_plugin.cpp b/src/ast/seq_decl_plugin.cpp
index 0dc9094ca..b9ed08cf7 100644
--- a/src/ast/seq_decl_plugin.cpp
+++ b/src/ast/seq_decl_plugin.cpp
@@ -1220,6 +1220,44 @@ app* seq_util::str::mk_is_empty(expr* s) const {
     return m.mk_eq(s, mk_empty(get_sort(s)));
 }
 
+unsigned seq_util::str::min_length(expr* s) const {
+    SASSERT(u.is_seq(s));
+    expr_ref_vector es(m);
+    unsigned result = 0;
+    get_concat_units(s, es);
+    for (expr* arg : es) {
+        if (is_unit(arg))
+            result++;
+    }
+    return result;
+}
+
+unsigned seq_util::re::min_length(expr* r) const {
+    SASSERT(u.is_re(r));
+    expr* r1 = nullptr, *r2 = nullptr, *s = nullptr;
+    if (is_empty(r))
+        return UINT_MAX;
+    if (is_concat(r, r1, r2)) {
+        unsigned l1 = min_length(r1);
+        if (l1 == UINT_MAX)
+            return l1;
+        unsigned l2 = min_length(r2);
+        if (l2 == UINT_MAX)
+            return l2;
+        return l1 + l2;
+    }
+    if (m.is_ite(r, s, r1, r2)) 
+        return std::min(min_length(r1), min_length(r2));
+    if (is_diff(r, r1, r2))
+        return min_length(r1);
+    if (is_union(r, r1, r2)) 
+        return std::min(min_length(r1), min_length(r2));
+    if (is_intersection(r, r1, r2)) 
+        return std::max(min_length(r1), min_length(r2));
+    if (is_to_re(r, s)) 
+        return u.str.min_length(s);
+    return 0;
+}
 
 sort* seq_util::re::to_seq(sort* re) {
     (void)u;
diff --git a/src/ast/seq_decl_plugin.h b/src/ast/seq_decl_plugin.h
index 46cc27fe4..e54db6567 100644
--- a/src/ast/seq_decl_plugin.h
+++ b/src/ast/seq_decl_plugin.h
@@ -394,6 +394,8 @@ public:
         void get_concat_units(expr* e, expr_ref_vector& es) const;
         expr* get_leftmost_concat(expr* e) const { expr* e1, *e2; while (is_concat(e, e1, e2)) e = e1; return e; }
         expr* get_rightmost_concat(expr* e) const { expr* e1, *e2; while (is_concat(e, e1, e2)) e = e2; return e; }
+
+        unsigned min_length(expr* s) const;
     };
 
     class re {
@@ -456,6 +458,7 @@ public:
         bool is_loop(expr const* n, expr*& body, unsigned& lo);
         bool is_loop(expr const* n, expr*& body, expr*& lo, expr*& hi);
         bool is_loop(expr const* n, expr*& body, expr*& lo);
+        unsigned min_length(expr* r) const;
     };
     str str;
     re  re;
diff --git a/src/smt/seq_regex.cpp b/src/smt/seq_regex.cpp
index d355d210f..51ca4ccf8 100644
--- a/src/smt/seq_regex.cpp
+++ b/src/smt/seq_regex.cpp
@@ -100,6 +100,32 @@ namespace smt {
             m_to_propagate.push_back(lit);
     }
 
+       
+    // s in R[if(p,R1,R2)] & p => s in R[R1]
+    // s in R[if(p,R1,R2)] & ~p => s in R[R2]
+
+    bool seq_regex::unfold_cofactors(expr_ref& r, literal_vector& conds) {
+        expr_ref cond(m), tt(m), el(m);
+        while (seq_rw().has_cofactor(r, cond, tt, el)) {
+            rewrite(cond);
+            literal lcond = th.mk_literal(cond);
+            switch (ctx.get_assignment(lcond)) {
+            case l_true: 
+                conds.push_back(~lcond);
+                r = tt;
+                break;
+            case l_false:
+                conds.push_back(lcond);
+                r = el;
+                break;
+            case l_undef:
+                ctx.mark_as_relevant(lcond);
+                return false;
+            }
+            rewrite(r);
+        }
+        return true;
+    }
 
     /**
      * Propagate the atom (accept s i r)
@@ -144,79 +170,30 @@ namespace smt {
         case l_false:
             break;
         }
-       
-        // s in R[if(p,R1,R2)] & p => s in R[R1]
-        // s in R[if(p,R1,R2)] & ~p => s in R[R2]
 
-        // TBD combine the two places that perform co-factor rewriting
-        expr_ref cond(m), tt(m), el(m);
-        if (seq_rw().has_cofactor(r, cond, tt, el)) {
-            rewrite(cond);
-            literal lcond = th.mk_literal(cond), next_lit;
-            switch (ctx.get_assignment(lcond)) {
-            case l_true: {
-                rewrite(tt);
-                literal lits[2] = { lit, lcond };
-                next_lit = th.mk_literal(sk().mk_accept(s, i, tt));
-                th.propagate_lit(nullptr, 2, lits, next_lit);
-                return true;
-            }
-            case l_false: {
-                rewrite(el);
-                next_lit = th.mk_literal(sk().mk_accept(s, i, el));
-                literal lits[2] = { lit, ~lcond };
-                th.propagate_lit(nullptr, 2, lits, next_lit);
-                return true;
-            }
-            case l_undef:
-                ctx.mark_as_relevant(lcond);
-                return false;
-            }
-        }
+        expr_ref head(m), d(r, m), i_next(m);        
+        literal_vector conds;
+
+        if (!unfold_cofactors(d, conds)) 
+            return false;
 
         // s in R & len(s) > i => tail(s,i) in D(nth(s, i), R)
-        expr_ref head(m), d(m), i_next(m);
         head = th.mk_nth(s, i);        
         i_next = a().mk_int(idx + 1);
-        d = seq_rw().derivative(head, r);
+        d = seq_rw().derivative(head, d);
         if (!d) 
             throw default_exception("unable to expand derivative");
 
-        // immediately expand a co-factor here so that condition on the character
-        // is expanded.
-
-        literal_vector conds;
-        while (seq_rw().has_cofactor(d, cond, tt, el)) {
-            rewrite(cond);
-            literal lcond = th.mk_literal(cond);
-            bool is_undef = false;
-            switch (ctx.get_assignment(lcond)) {
-            case l_true: 
-                conds.push_back(~lcond);
-                d = tt;
-                break;
-            case l_false:
-                conds.push_back(lcond);
-                d = el;
-                break;
-            case l_undef:
-                ctx.mark_as_relevant(lcond);
-                d = m.mk_ite(cond, tt, el);
-                is_undef = true;
-                break;
-            }
-            if (is_undef)
-                break;
-            rewrite(d);
-        }
+        literal acc_next = th.mk_literal(sk().mk_accept(s, i_next, d));
         if (conds.empty()) {
-            th.add_axiom(~lit, len_s_le_i, th.mk_literal(sk().mk_accept(s, i_next, d)));
+            th.add_axiom(~lit, len_s_le_i, acc_next);
         }
         else {
             conds.push_back(~lit);
             conds.push_back(len_s_le_i);
-            conds.push_back(th.mk_literal(sk().mk_accept(s, i_next, d)));
-            for (literal lit : conds) ctx.mark_as_relevant(lit);
+            conds.push_back(acc_next);
+            for (literal c : conds) 
+                ctx.mark_as_relevant(c);
             ctx.mk_th_axiom(th.get_id(), conds.size(), conds.c_ptr());
         }
         TRACE("seq", tout << "head " << head << "\n";
@@ -226,15 +203,12 @@ namespace smt {
 
     /**
      * Put a limit to the unfolding of s. 
-     * TBD: 
-     * Given an Regex R, we can compute the minimal length string accepted by R
-     * A strong analysis could compute the minimal length of s to be accepted by R
-     * For example if s = x + "abc" and R = .* ++ "def" . .**, then minimal length
-     * of s is 6.
-     * Limiting the depth of unfolding s below such lengths is not useful.
      */
 
     bool seq_regex::block_unfolding(literal lit, unsigned i) {
+        expr* s = nullptr, *idx = nullptr, *r = nullptr;
+        expr* e = ctx.bool_var2expr(lit.var());
+        VERIFY(sk().is_accept(e, s, idx, r));
         expr* t = nullptr;
         if (i > th.m_max_unfolding_depth && 
             th.m_max_unfolding_lit != null_literal && 
diff --git a/src/smt/seq_regex.h b/src/smt/seq_regex.h
index aa2b66b69..be88fee5e 100644
--- a/src/smt/seq_regex.h
+++ b/src/smt/seq_regex.h
@@ -61,6 +61,8 @@ namespace smt {
 
         expr_ref unroll_non_empty(expr* r, expr_mark& seen, unsigned depth);
 
+        bool unfold_cofactors(expr_ref& r, literal_vector& conds);
+
     public:
 
         seq_regex(theory_seq& th);
diff --git a/src/smt/theory_seq.cpp b/src/smt/theory_seq.cpp
index d9ec1c9a3..8cb98b479 100644
--- a/src/smt/theory_seq.cpp
+++ b/src/smt/theory_seq.cpp
@@ -3493,8 +3493,10 @@ bool theory_seq::should_research(expr_ref_vector & unsat_core) {
     }
     if (k_min < UINT_MAX) {
         m_max_unfolding_depth++;
-        IF_VERBOSE(1, verbose_stream() << "(smt.seq :increase-length " << mk_pp(s_min, m) << " " << 2*k_min << ")\n");
-        add_length_limit(s_min, 2*k_min, false);
+        k_min *= 2;
+        k_min = std::max(m_util.str.min_length(s_min), k_min);
+        IF_VERBOSE(1, verbose_stream() << "(smt.seq :increase-length " << mk_pp(s_min, m) << " " << k_min << ")\n");
+        add_length_limit(s_min, k_min, false);
         return true;
     }
     else if (has_max_unfolding) {