The function takes a string as input and performs the following operations:
- If the input string contains the letter 'e', it replaces all instances of 'e' with 'v'.
- Otherwise, it returns the input string as is.

[CODE]:
```python
def f(x):
    if 'e' in x:
        return x.replace('e', 'v')
    else:
        return x
```

Note: This description is based on the limited experiment results provided. It is recommended to test the function on a larger range of inputs to refine the description further.