From c7e7b40d40399fd98aa0b7716e53e7e92d12f2bd Mon Sep 17 00:00:00 2001
From: CEisenhofer <clemens.eisenhofer@tuwien.ac.at>
Date: Wed, 8 Apr 2026 09:27:46 +0200
Subject: [PATCH] Fix

---
 src/ast/euf/euf_sgraph.cpp  | 3 ++-
 src/ast/euf/euf_snode.h     | 4 +++-
 src/smt/seq/seq_nielsen.cpp | 5 ++++-
 3 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/ast/euf/euf_sgraph.cpp b/src/ast/euf/euf_sgraph.cpp
index 5768d7aec..cd975c328 100644
--- a/src/ast/euf/euf_sgraph.cpp
+++ b/src/ast/euf/euf_sgraph.cpp
@@ -47,7 +47,6 @@ namespace euf {
     snode_kind sgraph::classify(expr* e) const {
         if (!is_app(e))
             return snode_kind::s_var;
-
         if (m_seq.str.is_empty(e))
             return snode_kind::s_empty;
 
@@ -339,6 +338,8 @@ namespace euf {
         compute_metadata(n);
         compute_hash_matrix(n);
         m_nodes.push_back(n);
+        SASSERT(!n->is_char_or_unit() || m_seq.str.is_unit(n->get_expr()));
+
         if (e) {
             unsigned eid = e->get_id();
             m_expr2snode.reserve(eid + 1, nullptr);
diff --git a/src/ast/euf/euf_snode.h b/src/ast/euf/euf_snode.h
index b2241f0ee..1f8d3903b 100644
--- a/src/ast/euf/euf_snode.h
+++ b/src/ast/euf/euf_snode.h
@@ -27,6 +27,7 @@ Author:
 #include "util/region.h"
 #include "ast/ast.h"
 #include "ast/seq_decl_plugin.h"
+#include "ast/rewriter/seq_axioms.h"
 
 namespace euf {
 
@@ -88,8 +89,9 @@ namespace euf {
             n->m_kind = k;
             n->m_id = id;
             n->m_num_args = num_args;
-            for (unsigned i = 0; i < num_args; ++i)
+            for (unsigned i = 0; i < num_args; ++i) {
                 n->m_args[i] = args[i];
+            }
             return n;
         }
 
diff --git a/src/smt/seq/seq_nielsen.cpp b/src/smt/seq/seq_nielsen.cpp
index cf34e00de..2d857efea 100644
--- a/src/smt/seq/seq_nielsen.cpp
+++ b/src/smt/seq/seq_nielsen.cpp
@@ -267,6 +267,7 @@ namespace seq {
     }
 
     void nielsen_node::apply_subst(euf::sgraph& sg, nielsen_subst const& s) {
+        SASSERT(!s.m_var->is_char_or_unit() || s.m_replacement->is_char_or_unit());
         SASSERT(s.m_var);
         SASSERT(s.m_replacement != nullptr);
         for (auto &eq : m_str_eq) {
@@ -294,9 +295,11 @@ namespace seq {
 
         if (s.is_char_subst()) {
             ast_manager& m = graph().get_manager();
+            expr* var_c_expr = s.m_var->arg(0)->get_expr();
+            expr* repl_c_expr = s.m_replacement->arg(0)->get_expr();
             add_constraint(
                 constraint(
-                    m.mk_eq(s.m_var->get_expr(), s.m_replacement->get_expr()), s.m_dep, m));
+                    m.mk_eq(var_c_expr, repl_c_expr), s.m_dep, m));
 
             if (m_char_ranges.contains(var_id)) {
                 auto range = m_char_ranges.find(var_id); // copy exactly