Figure 4.1 shows a sample integrand function. This
function was previously presented in Figure 3.1. It
returns the value of 
for a given x, y, and
z value (or, rather, for a 3-dimensional 
value).
The integrand function is called by PARINT many times during the
evaluation of an integral. Each time, the appropriate value
is passed to the function, and the function is expected to calculate
and return in a parameter the appropriate 
value.
