The function replaces each input letter with the letter that is two positions ahead of it in the English alphabet, wrapping around from "z" to "a" when needed.

[CODE]: Here's a simple implementation in Python:

```python
def f(x):
    result = ""
    for char in x:
        new_char = chr(((ord(char) - ord('a') + 2) % 26) + ord('a'))
        result += new_char
    return result
```

Let's run one more test with a different set of inputs to confirm this description.

COMMAND: PYTHON(./temp/function.py "x" "y" "z")