WISHLIST

o xml features

o better simplification

o extend fn$ to proto objects

o can current rules of determining which formulas to convert to functions
  and which strings to perform interpolation on be improved?

o check number of function args in addition to the number of ( to improve
  backref heuristic

o add named character vectors as replacement argument objects

o performance.  See:
  https://stat.ethz.ch/pipermail/r-help/2008-November/179261.html