approxD: Approximate Function Derivatives
In bsearchinger/ARS: Adaptive Rejection Sampling

approxD approximates the derivative of a specified function at a specified value using a central finite difference approach.

1	approxD(f, f_params = NULL, x, n = 1, h = sqrt(.Machine$double.eps))

`f`	The function to evaluate, as an expression.
`f_params`	A `list` of any associated parameters of `f`.
`x`	The value at which to evaluate the derivative of `f`.
`n`	The order of derivative to calculate, 1 or 2. Default is 1.
`h`	The finite difference to use in the approximation. The default value is `sqrt(.Machine$double.eps)` by which the absolute value of `x` will be multiplied unless `x` is 0.