+ ##
+ ## INPUTS:
+ ##
+ ## * ``f`` - The function whose divided differences we want.
+ ##
+ ## * ``xs`` - A vector containing x-coordinates. The length of `xs`
+ ## determines the order of the divided difference.
+ ##
+ ##
+ ## OUTPUTS:
+ ##
+ ## * ``dd`` - The divided difference f[xs(1), xs(2),...]
+ ##
+
+ order = length(xs) - 1;
+
+ if (order < 0)