f must return the value of the integrand at a given point.
f must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which
d01paf is called. Arguments denoted as
Input must
not be changed by this procedure.
Note: f should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by
d01paf. If your code inadvertently
does return any NaNs or infinities,
d01paf is likely to produce unexpected results.