private void verbNetClasses_SelectedIndexChanged(object sender, EventArgs e)
        {
            verbNetRoles.Items.Clear();

            if (verbNetClasses.SelectedIndex == -1)
            {
                return;
            }

            foreach (VerbNetEngine.ThematicRole role in SelectedVerbNetClass.GetThematicRoles(true))
            {
                verbNetRoles.Items.Add(role);
            }
        }