Comment #1 on issue 2430 by christian.muise: Expression Countable Set
http://code.google.com/p/sympy/issues/detail?id=2430
Why not use lambda's to make up a set's behaviour? If it's countable, then
there should be a mapping from the ints to an element in the set and vice
versa. A lambda can be passed in that checks whether or not a number
qualifies. Using the set of even numbers as an example:
```python
even_numbers = CountableSet(lambda x: 0 == x % 2)
```
Internally, the check_membership function is set to be the passed in
lambda function. Then you could do set intersection (a new lambda that
checks both lambdas in the intersection), union (a new lambda that checks
at least one lambda in the union), etc.
Additionally, you could accept an optional second argument that is a
function to yield elements in the set given an integer:
```python
even_numbers = CountableSet(lambda x: 0 == x % 2, generator_func = lambda
x: 2*x)
```
If you are only concerned with sets that contain ints, you can have a
default generator_func that just iterates through integers until one passes
the test (inefficient, but gets the job done).
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