Add various possible TODO's.

TODO.md

@@ -3,3 +3,54 @@
 ## Review Signal responsibilities.
 Signals should only be concerned with how to display an aspect.
 Move the knowledge of number of free blocks into a "translator" object.
+
+## Support 2 aperture, 4 aspect signals.
+Modern 4 aspect signals have only two apertures (lights) where the lower
+one can show green, red or yellow.
+
+This can be implemented with NeoPixels.
+See discussion in https://www.merg.org.uk/forum/viewtopic.php?f=120&t=12963&p=145040#p145040
+
+Keep the design with signals that know how to show a particular aspect.
+New `Light` class that interacts with a NeoPixel object that controls 
+the LED. 
+There may be several `Light` objects connected to a single NeoPixel object.
+E.g. the signal has one red light, one green light and one yellow light.
+Each of these interact with one NeoPixel. 
+Question is who has the reference to the other.
+Assume that the signal will only turn on one of these `Light` objects at
+any time.
+
+## Need more flexibility for obstacles anywhere.
+Currently, cannot describe an obstacle such as a trailing point beyond
+a block.
+
+Suggest redesigning BlockDistanceInput to only take one block input
+and another DistanceInput. 
+Then these can be chained to produce any number of blocks.
+Within the chain we can now insert DistanceWithCondition objects at any point.
+
+## Find block count on diverging route.
+If we have a facing point then the number of free blocks used by
+the signal depends on the route selected by the point.
+
+Example:
+``` 
+ChoiceDistanceInput(DigitalInput pointPosition, DistanceInput route1, DistanceInput route2);
+```
+
+## Review CANSIG design for ideas
+CANSIG uses a set of flags that control the signal.
+If any flags in the red set is set, signal shows red.
+Otherwise, if any flags in the yellow set is set, signal shows yellow.
+Otherwise, if any flags in the double yellow set is set, signal shows double yellow.
+Otherwise, signal shows green.
+
+Looks very flexible, but is it easy to use? 
+Review a few examples.
+Maybe a DistanceInput implementation could use this logic?
+
+## Direct setting aspect
+This is useful for usage by JMRI which calculates the distance itself.
+
+This requires the separation of responsibilities above.