Handle piecewise functions symbolically

Currently, the following happens:

``` python
In [1]: import sympy

In [2]: p = lambda n: 1 if n == 0 else 0

In [3]: n = sympy.Symbol('n', integer=True, nonnegative=True)

In [4]: p(n)
Out[4]: 0
```

So, instead, `p` should be defined as a piecewise function, somehow.
