fix #5674

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/ast/ast.h

@@ -1050,6 +1050,12 @@ public:
     */
     virtual bool is_value(app * a) const { return false; }
 
+
+    /**
+       \brief return true if the expression can be used as a model value.
+     */
+    virtual bool is_model_value(app* a) const { return is_value(a); }
+
     /**
        \brief Return true if \c a is a unique plugin value.
        The following property should hold for unique theory values:

src/ast/seq_decl_plugin.cpp

@@ -705,6 +705,17 @@ bool seq_decl_plugin::is_value(app* e) const {
     }
 }
 
+bool seq_decl_plugin::is_model_value(app* e) const {
+    if (is_app_of(e, m_family_id, OP_SEQ_EMPTY)) 
+        return true;
+    if (is_app_of(e, m_family_id, OP_STRING_CONST)) 
+        return true;
+    if (is_app_of(e, m_family_id, OP_SEQ_UNIT) &&
+        m_manager->is_value(e->get_arg(0))) 
+        return true;
+    return false;
+}
+
 bool seq_decl_plugin::are_equal(app* a, app* b) const {
     if (a == b) return true;
     // handle concatenations

src/ast/seq_decl_plugin.h

@@ -179,6 +179,8 @@ public:
 
     bool is_value(app * e) const override;
 
+    bool is_model_value(app* e) const override;
+
     bool is_unique_value(app * e) const override;
 
     bool are_equal(app* a, app* b) const override;