several propagation modes

• Sequential
  • rays follow the directed graph from node to node. if the next node is not hit, further propagationis dropped. interesting for imaging, collimation, optimizing of "simple" optical lens systems
• Semi-sequential
  • rays may bounce and traverse the graph in backward direction. if the next intended node is not hit, further propagationis dropped. interesting for ghost focus simulation
• non-sequential
  • rays do not follow a specific direction of the graph. Skipping of nodes may be allowed. interesting for stray-light analysis, flash-lamp pumping, beam dumps