void hexa()
{
   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);
   TGeoArb8 *arb1 = new TGeoArb8("arb1",20);
   arb1->SetVertex(0,30,0);
   arb1->SetVertex(1,20,-25);
   arb1->SetVertex(2,-15,-35);
   arb1->SetVertex(3,-25,0);
   arb1->SetVertex(4,30,0);
   arb1->SetVertex(5,20,-25);
   arb1->SetVertex(6,-15,-35);
   arb1->SetVertex(7,-25,0);
   TGeoArb8 *arb2 = new TGeoArb8("arb2",20);
   arb2->SetVertex(0,30,0);
   arb2->SetVertex(1,-25,0);
   arb2->SetVertex(2,-5,20);
   arb2->SetVertex(3,10,15);
   arb2->SetVertex(4,30,0);
   arb2->SetVertex(5,-25,0);
   arb2->SetVertex(6,-5,20);
   arb2->SetVertex(7,10,15);
   TGeoCompositeShape *cs = new TGeoCompositeShape("hexa", "arb1+arb2");
   TGeoVolume *vol = new TGeoVolume("HEXA", cs,med);
   vol->SetLineColor(kRed);
   top->AddNode(vol,1);
   geom->CloseGeometry();
   top->Draw("ogl");
}