Question: How to define unit vectors of curvilinear coordinates in Physics[Vectors] package

Dear all
I am using Physics[Vectors] package of Maple 17. I want to define an orthogonal curvilinear coordinates through alpha and beta independent variables. To define unit vectors of alpha and beta, I have to apply the derivatives of position vector r_ with respect to alpha and beta, respectively. Please help me to define the unit vectors in directions alpha and beta as derivatives of position vector r_ with respect to alpha and beta, respectively. Please see the below Code:

> restart;

> with(Physics[Vectors]);

> Setup(mathematicalnotation=true);

> r_:=X(alpha,beta)*_i+Y(alpha,beta)*_j+Z(alpha,beta)*_k;

The unit vectors in directions alpha and beta should be defined as:

_alpha=diff(r_,alpha)
_beta=diff(r_,beta)

It is worthwhile to mention that the following expression governs:

> diff(r_,alpha).diff(r_,beta)=0;

Please guide me.

Best wishes

Ali