Math

General
+ Improve overall performance.
+ Make something happen when progress is completed successfully.
+ Improve user feedback for Drag & Drop via keyboard.
- Progress only when right answer is given (detect errors).
- Fix bug: line segment drop area sometimes stays highlighted.
- Fix bug: line segment drop targets incorrectly highlighted when moving by key press.
+ Restrain draggin of line segments, symbols not too far from drop targets.
+ Add sound effects.
+ Improve images of faucet, balance, scissors.
+ Animate pouring of water, maybe also cutting by scissors and motion of balance.
+ Tools (scissors, balance, faucet) also selectable from a menu.

Compare3 Lesson
- Control movement from one problem type to another.
- Don't repeat colors or letters from one problem to the next.
- Don't repeat recently used problems.
- Add more problems for some comparisons.
- Mass comparison: fit into answer box.
- Fix drawing of mass parallelopipeds (ThreeDObjects).
- Fix behavior of line segments (drops getting rejected).
- Volume comparison: fit into answer box.
- Amount comparison: fit into answer box (maybe make wider and reposition).
- Amount comparison: refine appearance of object groups.
- Amount comparison: refine appearance of model (make pagelike).
- Fix bug: length comparisons can give spurious correct result when lengths touch end-to-end, or overlap completely.