TEveGeoNode* descend_extract(TGeoNode* node)
{
   // We only return something if:
   // - this is a node of interest;
   // - one of the daughters returns something of interest.

   const TString material("Silicon");

   TEveGeoNode *res = 0;

   TRegexp endcap_layers_RE = "SI0[5-9]";
   TRegexp endcap_layers_RE_2 = "SI1[0-9]";

   TGeoMedium *medium = node->GetVolume()->GetMedium();
   if (medium && material == medium->GetName())
   {
      node->GetVolume()->SetTransparency(90);
      medium->GetMaterial()->SetTransparency(90);

      // Node of interest - instantiate eve representation and return.
      res = new TEveGeoNode(node);
      res->SetMainTransparency(60);
      return res;
   }

   Int_t nd = node->GetNdaughters();
   for (Int_t i = 0; i < nd; ++i)
   {
      if (TString(node->GetDaughter(i)->GetName()).Contains(endcap_layers_RE)
          || TString(node->GetDaughter(i)->GetName()).Contains(endcap_layers_RE_2))
      {
          continue;
      }

      TEveGeoNode *ed = descend_extract(node->GetDaughter(i));

      if (ed)
      {
         if (res == 0) res = new TEveGeoNode(node);
         res->SetMainTransparency(60);
         res->AddElement(ed);
      }
   }

   return res;
}