The function appears to replace each vowel ('a', 'e', 'i', 'o', 'u') with the letter following it in the alphabet, and then appends 'z' at the end of the string.

[CODE]:
```python
def f_approx(x):
    vowels = {'a': 'b', 'e': 'f', 'i': 'j', 'o': 'p', 'u': 'v'}
    result = ""
    for c in x:
        if c.lower() in vowels:
            result += vowels[c.lower()]
        else:
            result += c
    return result + 'z'
```

I will test the function with more inputs to verify its behavior.

COMMAND: PYTHON(./temp/function.py "education" "intelligence" "congratulation" "approximation")