private void btnDelete_Click(object sender, EventArgs e)
 {
     if (lstInstances.SelectedItems.Count == 1)
     {
         var             it     = lstInstances.SelectedItems[0];
         ISymbolInstance symRef = (ISymbolInstance)it.Tag;
         _comp.RemoveSymbolInstance(symRef);
         lstInstances.Items.Remove(it);
     }
 }