The function f(x) seems to alternate the second character of a string with 'o', 'd', or '!'. The character is replaced with 'o' if the position is a vowel, 'd' if the position is a consonant, and it remains '!' if the position is already '!' or any other non-alphabetical character.

[CODE]: 
```python
def f_approx(x):
    result = []
    for idx, char in enumerate(x):
        if idx % 2 == 1:
            if char.lower() in "aeiou":
                result.append("o")
            elif char.lower() in "bcdfghjklmnpqrstvwxyz":
                result.append("d")
            else:
                result.append("!")
        else:
            result.append(char)
    return "".join(result)
```

Keep in mind that this is an approximation, and there might be some edge cases not covered by this description or implementation.