TODO
====
- Optionally leaving out the * for multiplication.
- When navigating to a history entry and executing it again, without first changing it, move it on top of the history instead of creating a new history entry.
- Context-sensitive KeyState menus which only display those operations which are currently valid.
- plot() should accept a general expression involving x as its first parameter.
- Anti-aliasing of the plotted curve to make it look smoother.
- A two-dimensional plotter, for functions with two parameters, that maps function values to colour or grayscale values. Perhaps call it map().
- Rewrite Expr and Symbol to use a stack-based "virtual machine" - the speed gain should be useful especially for map().
- Make the plot screen interactive: user can move around and zoom (both plot() and map()).

- Add unit tests, get good test coverage.
- Fix bugs with function re-definition.
- Support pointer for pushing buttons (for touch-screen mobile phones).
- Improve usability (how?).
- Support different key layouts (e.g. for Nokia E61).

- Improve web page.
- Setup mailing list.
- Declare open-source status on web page, link to svn & mailing list.

DONE
====
- Only plot when the user presses the fire button.
- New release including the plot functionality (0.1.2).

IDEAS
=====
- Dimensions, unit conversion (300m/s -> km/h; sin(60deg); 100ft^2 -> m^2).
- Contextual help for functions.
- Allow custom function and parameter names (how?) (e.g.: BMI(weight, height) := weight/height^2). 
- Additional, optional function packages.