The function reverses the case of each letter in the input string, leaving non-letter characters unchanged.

[CODE]:
```python
def f(x):
    result = ""
    for char in x:
        if char.isalpha():
            if char.isupper():
                result += char.lower()
            else:
                result += char.upper()
       else:
            result += char
    return result
```

This code will approximate the behavior of the function by reversing the case of each letter while leaving non-letter characters unchanged.