Sorry. I have answered my own question. The addition of alacarte enabled me to add a menu item.
Very good, J. Tarr.
Good work, J. Tarr. That does it. I am curious about the way Maple works here. Once the directional derivative is defined, entering a vector, coordinates or other parameters requires that you call the directional derivative function again and presumably update it. I guess there is no way to call the directional derivative function just once (after you have defined a vector and point)? (I did try this and got an "unknown coordinate system: [4, 5, 6]" error.)
So, a numerical directional derivative for functions of 3 variables is only obtainable with a hyperplane where the gradient is constant? Giving one variable an exponent of 2 in your example function gives me a directional derivative which is a function of that variable. (Of course, a control-click (right click) allows evaluation at any point.)
Thanks, but all these methods give me functions of x, y, z. I wanted the directional derivative evaluated at a particular point with numerical rectangular coordinates, an answer something like 3 or sqrt(5) or Pi.