This should be a simple question. But sadly I couldn't figure it out on my own
:-(
I know how to get the multivariate Taylor series expansion of a function f of arguments X[1] and X[2], about zero, and to, say, the third order,
mtaylor( f(X[1],X[2]), [X[1],X[2]], 3 );
f(0, 0) + D[1](f)(0, 0) X[1] + D[2](f)(0, 0) X[2]
1  ...