[Maxima] Bigfloat routine for Incomplete Beta

[Richard Fateman]

You could try doing some or all of the calculation in rational numbers, and
converting
to bigfloat at the end.  

This might be slower.

Another way, non-portable, is to use the MPFR library for the elementary
operations.  
The library itself is
fairly portable, but the lisp-to-MPFR interface varies from one common lisp
to another.

MPFR may already have a gamma function that can be used directly from the
library..

http://cvs.sourceforge.jp/cgi-bin/viewcvs.cgi/pf3gnuchains/mpfr/gamma.c