Defining a function with vectors

[Quadratica]

I'm trying to define a function to calculate the cross product of two vectors

cross([A,B,C],[C,B,A]) I give it the name cp

when I define it all that happens is it replaces A,B,C with some number values and the tick boxes to show that it accepts and recognises A,B,C as variable arguments do not show up.

Other definitions like sin(A+B/C*D) work as expected and can be used in the CAS screen.

any help is appreciated as I'm doing lots of vector work lately and looking for shortcuts.