draw:formula issues

[Chris Nokleberg <chris@sixlegs.com>]

The BNF for draw:formula includes these functions:

  unary_function = 'abs'|'sqrt'|'sin'|'cos'|'tan'|'atan'|'atan2'
  binary_function = 'min'|'max'
  ternary_function = 'if'

The functions are not specified anywhere. Consistent rendering of custom
shapes across implementations require that these functions be specified.

Some specific questions:

- Do the trig functions use degrees or radians?
- Shouldn't atan2 be a binary function?
- What is the "if" function?
  
  In VML, if(v, P1, P2) is the same as (v > 0 ? P1 : P2).
  See http://www.w3.org/TR/1998/NOTE-VML-19980513#_Toc416858392

Thanks,
Chris