The function transforms each character of the input string by applying a specific operation. However, if the input string consists only of digits, the function returns the input string unchanged.

[CODE]:
```
def f(x):
    if x.isdigit():
        return x
    else:
        transformed_string = ""
        for char in x:
            transformed_string += <transformation_operation(char)>
        return transformed_string
```

In the above code, `<transformation_operation(char)>` represents the specific operation being applied to each character of the input string. We would need further experiments to determine the exact nature of this operation.