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)
I work interactively a lot. And I rarely remember all the methods of objects. So in IPython I use the `?` all the time. Not if I have a `Vector` instance and wonder what the hell `as_coeff_Mul` does, then the quickest way to know is to simply type `vec.as_coeff_Mul?` and it tells me. But if there is no doc string then I have to start searching the internet or looking in other classes or grepping the sympy code, which is a complete waste of time. So, yes, I find docstrings to all public methods of each class very necessary. Also in most sophisticated IDES (or even my vim extenstions for python) I can select any method and have the docstring pop up. But that only works if the method has a doc string.