Directional Derivative

[salvomic]

hi all,
would like anybody to help me to get a program for Directional Derivative?
Starting with this: it's the dot product between Gradient of v and person of u, ( {tex} \( \bigtriangledown f \cdot \overrightarrow{v} \) {/tex} ) so
dot (grad(u) , v/normalize(v)) could be good, but if use it with "Define" it doesn't works, as "dot" (lowercase) is always changed in "DOT" (uppercase) and the function gives error "dot vector Error: bad argument value"...

(I'm using CAS)

Thank you in advance!

Salvo