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)
Why check isinstance twice, you could:

```python
if isinstance(other, Del):
    # do somethign
elif isinstance(other, Vector):
   #do something else
else:
    raise TypeError('...')
```