code: def fprob(dfnum, dfden, F):
      
      p = betai(0.5 * dfden, 0.5 * dfnum, dfden / float(dfden + dfnum * F))
      return p
docs: Returns the (1-tailed) significance level (p-value) of an F
  statistic given the degrees of freedom for the numerator (dfR-dfF) and
  the degrees of freedom for the denominator (dfF).
  
  Usage:   lfprob(dfnum, dfden, F)   where usually dfnum=dfbn, dfden=dfwn