```
In [0]: _find_first_symbol(Sum(sqrt(x), (x, 1, 3)).function)

Out[0]: x
In [1]: _find_first_symbol(Sum(n*sqrt(x), (x, 1, 3)).function)

Out[1]: x
In [2]: _find_first_symbol(Integral(z*sqrt(x), (x, 1, 3)).function)
Out[2]: x
```
For the moment ```free_symbols``` is much better than ```.atoms()```.