gsldif input parametersThe argument to the functions with mnemonics of backward,
central or forward is a pair (f,x),
where f is the virtual machine code for a real valued function
of a real variable, and x is the input to f where the
derivative is sought. Real numbers are represented according to
math.
The argument to the functions with mnemonics of t_backward,
t_central or t_forward is a pair ((f,t),x),
where f and x are as above, and t is a tolerance represented
as a floating point number. The tolerance is passed through
to the GNU Scientific library (GSL) differentiation routines.
When no tolerance is specified, the default is 1.0e-8.