Index: tol/btol/set_type/setgra.cpp
===================================================================
--- tol/btol/set_type/setgra.cpp	(revision 85745be4164302d4af3d44b6495f26b2fef0c5c3)
+++ tol/btol/set_type/setgra.cpp	(revision b4f1dbfc597447653c4fc8846c72a1b1154122c0)
@@ -124,4 +124,10 @@
 
 
+  void ErrCannotModify(const BText& member, const BSyntaxObject* constant) 
+  { 
+    Error(I2("Cannot modify <","No se puede modificar <") + member +
+          I2("> of constant object ","> del objeto constante ") + 
+          constant->Identify()); 
+  }
 
 //--------------------------------------------------------------------
@@ -162,4 +168,8 @@
     bool incrementalIndexByName = false;
     BUserSet* uSet1 = USet(lst->Car()); lst = lst->Cdr();
+    if(uSet1->IsConstant()) { 
+      ErrCannotModify("value",uSet1);
+      return(NULL);
+    }
     BUserSet* uSet2 = USet(lst->Car()); lst = lst->Cdr();
     if(lst && lst->Car())
@@ -209,4 +219,8 @@
     bool incrementalIndexByName = false;
     BUserSet* uSet = USet(lst->Car()); lst = lst->Cdr();
+    if(uSet->IsConstant()) { 
+      ErrCannotModify("value",uSet);
+      return(NULL);
+    }
     BUserDat* uDat = UDat(lst->Car()); lst = lst->Cdr();
     bool do_deletion = true;