FEM: node sets, return sorted with unique values

src/Mod/Fem/femsolver/writerbase.py

@@ -123,6 +123,12 @@ class FemInputWriter():
         self.femelement_edges_table = {}
         self.femelement_count_test = True
 
+    # use set for node sets to be sure all nodes are unique
+    # use sorted to be sure the order is the same on different runs
+    # be aware a sorted set returns a list, because set are not sorted by default
+    #     - done in return value of meshtools.get_femnodes_by_femobj_with_references
+    # might be appropriate for element sets too
+
     def get_constraints_fixed_nodes(self):
         # get nodes
         for femobj in self.fixed_objects: