TODO
====
- Various colour gradients for map() to select from
- Use |x| instead of abs(x), and perhaps also the usual notation for floor, ceil, int
- Externalise strings and make it translatable
- Want CompiledFunction#xyFragment to use
        "x+x+y" ==> "(x+x)+y"
    but "x+y+y" ==> "x+(y+y)" (use associativity) 
    and "x+y+x" ==> "(x+x)+y" (use commutativity)
    and similarly for multiplication.
  BUT: (1e300*1e300)*1e-300 == +Inf whereas 1e300*(1e300*1e-300) == 1e300. Ignore? 
- Mark errors in history items (either same way as in input line, or "X" symbol next to it)
- Perhaps check whether glyph for \u00d7 (×, multiplication symbol) differs from symbol for "x"
  and also differs from box symbol; in that case use \u00d7 instead of '*' 
- 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.
- Make full screen optional.
- 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.
- Find a way of displaying "Menu" and "Backspace" commands (in non-fullscreen mode) that don't prevent
  the Back command from working in the menus (at least in SE K310 emulator)

- 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")
- A way of finding out whether font contains letter pi: draw letter pi on an image; draw letter rho on
  an image; compare images pixel-by-pixel; if equal, pi not supported, otherwise, pi supported.
  Do this once and store the result in config.
- Automatically insert '*' in between when inserting a name (such as "f", "sin") after another name
- Get rid of String.valueOf calls in CalcCanvas whenever possible, and replace by passing line, len,
  to avoid copying of char array
- Ability to redefine a function. Easiest way: when input line is 'f(' and user presses clear, only delete '(' not whole line.
- map() "fragments" the function into parts that only involve x and those that only involve y, and
  evaluates those fragments at all the x-values respectively y-values first, thus bringing down the
  time taken for those fragments from O(w*h) to O(w+h) where w=width, h=height.
- e^(something) should yield code [(something); EXP] rather than [CONST_E; (something); POWER]
  (currently, opcode EXP is never generated)

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()
- Differentiate, integrate, find extrema, find zeros (numerically).
- Symbolic differentiation.
- % operator can be very confusing to non-programmers, since in calculators it usually means "percent"
  replace by "mod"?
- Complex numbers
- Fractals (Mandelbrot, Julia)