if FDR is zero, a log or logistic transform will fail; we add the small
positive number fudge to avoid this
zthresh
for association scores greater than this value, a hard value
of FDR 0 is assigned
maxch
the model for the functional relationship between association and FDR
is subset to observations for which association chisq score is no
greater than 1.1*maxch
...
arguments passed to s for the smooth
model relating association score to FDR at selected quantiles
of the association score distribution