The cdfchi function comes from dcdflib.c-1.1, a free library of
routines for cumulative distribution functions.  The library is
distributed by the M. D. Anderson Cancer Center's Department of
Biomathematics and Biostatistics.  Source is available from
  http://odin.mdacc.tmc.edu/anonftp/

The dcdflib cdfchi function is both faster and more accurate than the
prbf approximation used in early testing.
