From 71e239c08efb201640bb4c0224e87a5c03291964 Mon Sep 17 00:00:00 2001
From: Nikolaj Bjorner <nbjorner@microsoft.com>
Date: Sun, 6 Jan 2019 11:49:47 -0800
Subject: [PATCH] fix #2061

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>
---
 src/ast/rewriter/seq_rewriter.cpp | 35 +++++++++++++++++++++++++++++++
 src/ast/rewriter/seq_rewriter.h   |  1 +
 src/qe/qe_mbp.cpp                 |  4 ++--
 3 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index d46f234e2..68ddad07d 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -459,6 +459,9 @@ br_status seq_rewriter::mk_app_core(func_decl * f, unsigned num_args, expr * con
     case OP_SEQ_AT:
         SASSERT(num_args == 2);
         return mk_seq_at(args[0], args[1], result); 
+    case OP_SEQ_NTH:
+        SASSERT(num_args == 2);
+        return mk_seq_nth(args[0], args[1], result); 
     case OP_SEQ_PREFIX: 
         SASSERT(num_args == 2);
         return mk_seq_prefix(args[0], args[1], result);
@@ -877,6 +880,38 @@ br_status seq_rewriter::mk_seq_at(expr* a, expr* b, expr_ref& result) {
     return BR_DONE;
 }
 
+br_status seq_rewriter::mk_seq_nth(expr* a, expr* b, expr_ref& result) {
+    zstring c;
+    rational r;
+    if (!m_autil.is_numeral(b, r) || !r.is_unsigned()) {
+        return BR_FAILED;
+    }
+    unsigned len = r.get_unsigned();
+
+    expr_ref_vector as(m());
+    m_util.str.get_concat_units(a, as);
+
+    for (unsigned i = 0; i < as.size(); ++i) {
+        expr* a = as.get(i), *u = nullptr;
+        if (m_util.str.is_unit(a, u)) {
+            if (len == i) {
+                result = u;
+                return BR_REWRITE1;
+            }            
+        }
+        else if (i > 0) {
+            SASSERT(len >= i);
+            result = m_util.str.mk_concat(as.size() - i, as.c_ptr() + i);
+            result = m().mk_app(m_util.get_family_id(), OP_SEQ_NTH, result, m_autil.mk_int(len - i));
+            return BR_REWRITE2;
+        }
+        else {
+            return BR_FAILED;
+        }
+    }
+    return BR_FAILED;
+}
+
 br_status seq_rewriter::mk_seq_index(expr* a, expr* b, expr* c, expr_ref& result) {
     zstring s1, s2;
     rational r;
diff --git a/src/ast/rewriter/seq_rewriter.h b/src/ast/rewriter/seq_rewriter.h
index 83793a594..25b8979fc 100644
--- a/src/ast/rewriter/seq_rewriter.h
+++ b/src/ast/rewriter/seq_rewriter.h
@@ -114,6 +114,7 @@ class seq_rewriter {
     br_status mk_seq_extract(expr* a, expr* b, expr* c, expr_ref& result);
     br_status mk_seq_contains(expr* a, expr* b, expr_ref& result);
     br_status mk_seq_at(expr* a, expr* b, expr_ref& result);
+    br_status mk_seq_nth(expr* a, expr* b, expr_ref& result);
     br_status mk_seq_index(expr* a, expr* b, expr* c, expr_ref& result);
     br_status mk_seq_replace(expr* a, expr* b, expr* c, expr_ref& result);
     br_status mk_seq_prefix(expr* a, expr* b, expr_ref& result);
diff --git a/src/qe/qe_mbp.cpp b/src/qe/qe_mbp.cpp
index a0e16727e..becaeb049 100644
--- a/src/qe/qe_mbp.cpp
+++ b/src/qe/qe_mbp.cpp
@@ -501,7 +501,7 @@ public:
         expr_ref val(m);
         model_evaluator eval(model);
         for (expr * f : fmls) {
-            VERIFY(model.is_true(f));
+            VERIFY(!model.is_false(f));
         }
         return true;
     }
@@ -657,7 +657,7 @@ public:
             other_vars.reset();
         }
         
-        SASSERT(eval.is_true(fml));
+        SASSERT(!eval.is_false(fml));
                 
         vars.reset ();
         vars.append(other_vars);