This looks good for now. Once this PR is in, we could use the `integer` function from here:

https://github.com/certik/csympy/blob/master/src/integer.h#L117

i.e. you would do something like:
```c++
RCP<Integer> i2 = integer(2);
...
assert(eq(gcd(*i2, *i4), integer(2)));
```
I can try to do this change once this PR is in.