@skirpichev this was working wrong in case like

```
In [4]: _has_only_supported_func(log(x)**sin(x), x)
Out[4]: True
```

I've fixed it, now. For me it was little tricky to implement.
Here's the output after the fix.

```
In [4]: _has_only_supported_func(log(x)**sin(x), x)
Out[4]: False

In [5]: _has_only_supported_func(exp(1/exp(1/(x-2)**2 - 1)), x)
Out[5]: True

In [6]: _has_only_supported_func(exp(x), x)
Out[6]: True

In [7]: _has_only_supported_func(exp(1/exp(1/(x-2)**2 - cos(x))), x)
Out[7]: False

In [8]: _has_only_supported_func(log(x)**exp(sin(x) + 1), x)
Out[8]: False
```