static Ref_t create_detector(LCDD& lcdd, xml_h e, SensitiveDetector sens)  {
  typedef vector<PlacedVolume> Placements;
  xml_det_t   x_det     = e;
  Material    vacuum    = lcdd.vacuum();
  int         det_id    = x_det.id();
  string      det_name  = x_det.nameStr();
  bool        reflect   = x_det.reflect(false);
  DetElement  sdet        (det_name,det_id);
  Assembly    assembly    (det_name);
  //Volume      assembly    (det_name,Box(10000,10000,10000),vacuum);
  Volume      motherVol = lcdd.pickMotherVolume(sdet);
  int         m_id=0, c_id=0, n_sensor=0;
  map<string,Volume> modules;
  map<string, Placements>  sensitives;
  PlacedVolume pv;

  assembly.setVisAttributes(lcdd.invisible());
  sens.setType("tracker");

  for(xml_coll_t mi(x_det,_U(module)); mi; ++mi, ++m_id)  {
    xml_comp_t x_mod   = mi;
    string     m_nam   = x_mod.nameStr();
    xml_comp_t trd     = x_mod.trd();
    double     posY;
    double     x1      = trd.x1();
    double     x2      = trd.x2();
    double     z       = trd.z();
    double     y1, y2, total_thickness=0.;
    xml_coll_t ci(x_mod,_U(module_component));
    for(ci.reset(), total_thickness=0.0; ci; ++ci)
      total_thickness += xml_comp_t(ci).thickness();
      
    y1 = y2 = total_thickness / 2;
    Volume  m_volume(m_nam, Trapezoid(x1, x2, y1, y2, z), vacuum);      
    m_volume.setVisAttributes(lcdd.visAttributes(x_mod.visStr()));

    for(ci.reset(), n_sensor=1, c_id=0, posY=-y1; ci; ++ci, ++c_id)  {
      xml_comp_t c       = ci;
      double     c_thick = c.thickness();
      Material   c_mat   = lcdd.material(c.materialStr());
      string     c_name  = _toString(c_id,"component%d");
      Volume     c_vol(c_name, Trapezoid(x1,x2,c_thick/2e0,c_thick/2e0,z), c_mat);

      c_vol.setVisAttributes(lcdd.visAttributes(c.visStr()));
      pv = m_volume.placeVolume(c_vol,Position(0,posY+c_thick/2,0));
      if ( c.isSensitive() ) {
        sdet.check(n_sensor > 2,"SiTrackerEndcap2::fromCompact: "+c_name+" Max of 2 modules allowed!");
        pv.addPhysVolID("sensor",n_sensor);
        c_vol.setSensitiveDetector(sens);
        sensitives[m_nam].push_back(pv);
        ++n_sensor;
      }
      posY += c_thick;
    }
    modules[m_nam] = m_volume;
  }
  
  for(xml_coll_t li(x_det,_U(layer)); li; ++li)  {
    xml_comp_t  x_layer(li);
    int l_id    = x_layer.id();
    int mod_num = 1;

    for(xml_coll_t ri(x_layer,_U(ring)); ri; ++ri)  {
      xml_comp_t x_ring = ri;
      double r        = x_ring.r();
      double phi0     = x_ring.phi0(0);
      double zstart   = x_ring.zstart();
      double dz       = x_ring.dz(0);
      int    nmodules = x_ring.nmodules();
      string m_nam    = x_ring.moduleStr();
      Volume m_vol    = modules[m_nam];
      double iphi     = 2*M_PI/nmodules;
      double phi      = phi0;
      Placements& sensVols = sensitives[m_nam];

      for(int k=0; k<nmodules; ++k) {
        string m_base = _toString(l_id,"layer%d") + _toString(mod_num,"_module%d");
        double x = -r*std::cos(phi);
        double y = -r*std::sin(phi);
        DetElement module(sdet,m_base+"_pos",det_id);
        pv = assembly.placeVolume(m_vol,Transform3D(RotationZYX(0,-M_PI/2-phi,-M_PI/2),Position(x,y,zstart+dz)));
        pv.addPhysVolID("side",1).addPhysVolID("layer", l_id).addPhysVolID("module",mod_num);
        module.setPlacement(pv);
        for(size_t ic=0; ic<sensVols.size(); ++ic)  {
          PlacedVolume sens_pv = sensVols[ic];
          DetElement comp_elt(module,sens_pv.volume().name(),mod_num);
          comp_elt.setPlacement(sens_pv);
        }

        if ( reflect ) {
          pv = assembly.placeVolume(m_vol,Transform3D(RotationZYX(M_PI,-M_PI/2-phi,-M_PI/2),Position(x,y,-zstart-dz)));
          pv.addPhysVolID("side",-1).addPhysVolID("layer",l_id).addPhysVolID("module",mod_num);
          DetElement r_module(sdet,m_base+"_neg",det_id);
          r_module.setPlacement(pv);
          for(size_t ic=0; ic<sensVols.size(); ++ic)  {
            PlacedVolume sens_pv = sensVols[ic];
            DetElement comp_elt(r_module,sens_pv.volume().name(),mod_num);
            comp_elt.setPlacement(sens_pv);
          }
        }
        dz   = -dz;
        phi += iphi;
        ++mod_num;
      }
    }
  }
  pv = motherVol.placeVolume(assembly);
  pv.addPhysVolID("system",det_id);
  sdet.setPlacement(pv);
  return sdet;
}
static Ref_t create_detector(Detector& theDetector, xml_h e, SensitiveDetector sens)  {
  typedef vector<PlacedVolume> Placements;
  xml_det_t   x_det     = e;
  Material    vacuum    = theDetector.vacuum();
  int         det_id    = x_det.id();
  string      det_name  = x_det.nameStr();
  bool        reflect   = x_det.reflect(false);
  DetElement  sdet        (det_name,det_id);
  int         m_id=0, c_id=0, n_sensor=0;
  map<string, Volume> modules;
  map<string, Placements>  sensitives;
  PlacedVolume pv;

  // --- create an envelope volume and position it into the world ---------------------

  Volume envelope = dd4hep::xml::createPlacedEnvelope(theDetector, e, sdet);
  dd4hep::xml::setDetectorTypeFlag(e, sdet);

  if( theDetector.buildType() == BUILD_ENVELOPE ) return sdet;

  //-----------------------------------------------------------------------------------

  envelope.setVisAttributes(theDetector.invisible());
  sens.setType("tracker");

  // Build the sensor units
  // Loop over 'modules' as defined in the XML
  for(xml_coll_t mi(x_det,_U(module)); mi; ++mi, ++m_id) { 
    xml_comp_t x_mod   = mi;
    string     m_nam   = x_mod.nameStr();
    xml_comp_t trd     = x_mod.trd();
    double     posY;
    double     x1      = trd.x1();
    double     x2      = trd.x2();
    double     z       = trd.z();
    double     y1, y2, total_thickness=0.;
    xml_coll_t ci(x_mod, _U(module_component));
    for(ci.reset(), total_thickness=0.0; ci; ++ci)
      total_thickness += xml_comp_t(ci).thickness();
      
    y1 = y2 = total_thickness / 2;
    Volume m_volume(m_nam, Trapezoid(x1, x2, y1, y2, z), vacuum);      
    m_volume.setVisAttributes(theDetector.visAttributes(x_mod.visStr()));

    std::cout << m_nam << ", thickness=" << total_thickness << std::endl;

    // Loop over the module_components ('slices') in the 'module'
    // The first component (top in the XML) is placed at the 'bottom'
    for(ci.reset(), n_sensor=1, c_id=0, posY=-y1; ci; ++ci, ++c_id) {
      xml_comp_t c       = ci;
      double     c_thick = c.thickness();
      Material   c_mat   = theDetector.material(c.materialStr());
      string     c_name  = _toString(c_id, "component%d");
      Volume     c_vol(c_name, Trapezoid(x1,x2,c_thick/2e0,c_thick/2e0,z), c_mat);

      std::cout << " + sensor " << n_sensor << " " << c_name;

      c_vol.setVisAttributes(theDetector.visAttributes(c.visStr()));
      pv = m_volume.placeVolume(c_vol, Position(0, posY + c_thick/2, 0));
      if ( c.isSensitive() ) {
        sdet.check(n_sensor > 2, "SiTrackerEndcap::fromCompact: " + c_name + " Max of 2 modules allowed!");
	pv.addPhysVolID("sensor", n_sensor);
        c_vol.setSensitiveDetector(sens);
        sensitives[m_nam].push_back(pv);
	std::cout << " (" << n_sensor << " is sensitive) ";
        ++n_sensor;
      }
      std::cout << std::endl;
      posY += c_thick;
    }
    modules[m_nam] = m_volume;
  }
  // done building the 2 modules, of 12 layers each

  int mod_count[12] = {0};

  // Build now the detector itself
  // Loop over layers as defined in the XML
  for(xml_coll_t li(x_det, _U(layer)); li; ++li) {
    xml_comp_t x_layer(li);
    int l_id = x_layer.id();
    int ring_num = 0;
    
    std::cout << "Layer " << l_id << ":" << std::endl;

    // Loop over rings, as defined in the XML
    for(xml_coll_t ri(x_layer, _U(ring)); ri; ++ri) {
      xml_comp_t x_ring = ri;
      double r        = x_ring.r();
      double phi0     = x_ring.phi0(0);
      double zstart   = x_ring.zstart();
      double dz       = x_ring.dz(0);
      int    nmodules = x_ring.nmodules();
      string m_nam    = x_ring.moduleStr();
      Volume m_vol    = modules[m_nam];
      double iphi     = 2*M_PI/nmodules;
      double phi      = phi0;
      Placements& sensVols = sensitives[m_nam];

      // This driver version encodes the rings as layers and the
      // petals as modules, such that 'layer' 1 contains all innermost rings
      // and last 'layer' contains the outermost rings in the tracker
      // farthest away on z from the IP (unintuititive, but works)

      std::cout << " Ring " << ring_num << ":" << std::endl;

      // Loop over modules in each ring, modules are either type 1 or 2
      for(int k=0; k < nmodules; ++k) {

        double x = -r*std::cos(phi);
        double y = -r*std::sin(phi);

	for(int s=1-2*int(reflect); s<2; s+=1+int(reflect)){

	  string e_name = _toString(s, "side%d") + _toString(l_id, "_layer%d") + _toString(ring_num, "_ring%d") + _toString(k, "_sensor%d");
	  DetElement module(sdet, e_name, det_id);
	  pv = envelope.placeVolume(m_vol, Transform3D(RotationZYX(0,-M_PI/2-phi,-M_PI/2), Position(x, y, s*(zstart+dz) )));

	  pv.addPhysVolID("side", s).addPhysVolID("layer", ring_num).addPhysVolID("module", mod_count[ring_num] + k);
	  module.setPlacement(pv);

	  for(size_t ic=0; ic<sensVols.size(); ++ic) {
	    PlacedVolume sens_pv = sensVols[ic];
	    DetElement comp_elt(module, sens_pv.volume().name(), det_id);
	    comp_elt.setPlacement(sens_pv);
	    std::cout << "Name: " << e_name << "_" << sens_pv.volume().name() << std::endl;
	    std::cout << "  ID: side " << s << ", layer " << ring_num << ", module " << mod_count[ring_num] + k << ", sensor" << ic+1 << std::endl;
	  }
	}
        dz = -dz;
        phi += iphi;
      }
      mod_count[ring_num] += nmodules;
      ++ring_num;
    }
  }
  std::cout << "Number of modules per 'layer':" << std::endl;
  for(int ii=0; ii<12; ii++){
    std::cout << " mod_count[" << ii << "] = " << mod_count[ii] << std::endl;
  }

  return sdet;
}