For each character in the input string, if the character is a lowercase letter, it is replaced by the letter 2 positions ahead in the alphabet (circular, e.g., 'z' goes to 'b'). Other characters are unchanged. Digits and special characters are preserved as they are.

[CODE]: 
```python
def f(x):
    output = ""
    for char in x:
        if char.islower():
            output += chr(((ord(char) - ord('a') + 2) % 26) + ord('a'))
        else:
            output += char
    return output
```

Let me know if you want more tests to confirm or refine the description.