Basic vector code, without a coordinate system class

All calculations essentially occur in the same frame.
i, j, k and x, y, z are imported as base vectors and base scalars by default on importing sympy.vector.
delop is a Del operator instance.

Example:

>>> v = 3*i - j
>>> delop ^ v
0

(delop ^ v represents curl of v)
Take a look at `diff` and `Derivative` here: http://docs.sympy.org/latest/tutorial/calculus.html

I'm just wondering if that pattern should be applied here.