void hexa1()
{
   gSystem->Load("libGeom");
   TGeoManager *geom = new TGeoManager("geom", "irregular hexagone");
   TGeoMaterial *mat = new TGeoMaterial("Al", 26.98,13,2.7);
   TGeoMedium   *med = new TGeoMedium("MED",1,mat);
   TGeoVolume   *top = geom->MakeBox("TOP",med,50,50,50);
   gGeoManager->SetTopVolume(top);
   TGeoXtru *xtru = new TGeoXtru(2); // 2 = nb. of Z planes
   Double_t x[6] = {30,20,-15,-25,-5,10};
   Double_t y[6] = {0,-25,-35,0,20,15};
   xtru->DefinePolygon(6,x,y);
   Double_t zsection = 20.;
   Double_t shiftx = 0.;
   Double_t shifty = 0.;
   Double_t scale = 1.;
   xtru->DefineSection(0,-zsection, shiftx, shifty, scale);
   xtru->DefineSection(1, zsection, shiftx, shifty, scale);
   TGeoVolume *vol = new TGeoVolume("HEXA", xtru,med);
   vol->SetLineColor(kRed);
   top->AddNode(vol,1);
   geom->CloseGeometry();
   top->Draw("ogl");
}