TODO
====
- Get rid of String.valueOf calls in CalcCanvas whenever possible, and replace by passing line, len,
  to avoid copying of char array
- For plot and par, do not cache whole Image but only function values
- Better error handling: in various places, NullPointerException/OutOfMemoryError/ArrayIndexOutOfBounds etc.
  may be thrown e.g. if input is very long or device is out of memory.
- Ability to redefine a function. Easiest way: when input line is 'f(' and user presses clear, only delete '(' not whole line.
- Make full screen optional.
- Option to use unicode character for pi (unless there is some automatic way of finding out whether the Font
  contains it).
- Conditional (ternary) operator: f:=x<2?1:f(x-2)+f(x-1). Clearly, this makes recursive functions
  useful, so we would need to allow those.
- 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.
- Make plotting interruptible.
- Display progress bar when plotting.
- Optionally rotate the plot by 90 degrees (so the user has to rotate their phone), yielding a landscape orientation which may be better suited to many plots.
- Anti-aliasing of the plotted curve to make it look smoother.
- Make the plot screen interactive: user can move around and zoom (both plot() and map()).
- The plot labels should honour the "smart rounding" setting.

- Add unit tests, get good test coverage.
- 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
====
- Optionally leaving out the * for multiplication.
  Note, this sometimes makes things look strange - e.g. when f is a number not a function, then 'f(3)' is valid.
- Only plot when the user presses the fire button.
- A two-dimensional plotter, for functions with two parameters, that maps function values to colour or grayscale values. Perhaps call it map().
- map() plot should indicate values corresponding to white and black.
- Rewrite Expr and Symbol to use a stack-based "virtual machine" - the speed gain should be useful especially for map().
- plot() should accept a general expression involving x as its first parameter. Similar for map().
- Allow expressions like '1+1)*(2+2'
- Exit application on BACK key.
- Fix bugs with function re-definition.
- Parametric plot: par(f,g,t_min,t_max)  
- For par(), perhaps rename x to t to avoid confusion.
- Add [xy]{min,max} labels to par().
- markError should mark the whole token, not just a single character
- Make axes drawing optional in plot() and par().
- Make label drawing optional in plot(), par() and map().
- Option of unit aspect ratio for plot(), map() and par(). For map(), this changes its arity from 5 to 4.
- Perhaps change the way submenus are indicated to displaying a green triangle in the lower right corner.
- When cursor is inside a plot command, result line shows description of current parameter (e.g. "function of x")


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.

- Peephole optimizing and inlining before plot() and map()
- To optimize map(), write a peephole optimizer. Then, for each y, modify the program by replacing
  each occurrence of PAR_Y by a literal with the current value of y, and optimize the program.
  Evaluate it for each x. (In many cases, this may actually slow down things.)
- Differentiate, integrate, find extrema, find zeros (numerically).
- Symbolic differentiation.