sympy/sympy — pull request #1390 by scolobb (2012-06-28 20:48:27+00:00)
Title: Fix sympy.core.sets.set_sort_fn (related to Union)

Currently, the following fails in `master`
```
x = Symbol('x')
y = Symbol('y')
z = Symbol('z')

S.EmptySet | FiniteSet(x, FiniteSet(y, z))
```

I get:
```
ValueError: The argument '{y, z}' is not comparable.
```

This pull request fixes this problem and also changes `set_sort_fn` to use `default_sort_key`.