The function f(x) appears to perform the following steps on the input string x:
1. Reverse the order of the string.
2. For each character in the reversed string,
   - If it's an alphabet character, replace it with the character that is the same distance away from 'z' as the original character is from 'a'. (In other words, the sum of the positions of the original and new characters in the alphabet - 1 is 25.)
   - If it's a non-alphanumeric character, keep it unchanged.

[CODE]:
```python
def approx_function(x: str) -> str:
    reversed_str = x[::-1]
    result = ''
    
    for char in reversed_str:
        if char.isalpha():
            new_char = chr(ord('a') + (25 - (ord(char.lower()) - ord('a'))))
            result += new_char if char.islower() else new_char.upper()
        else:
            result += char
    
    return result
```
Please note that this Python code is an approximation of the function's behavior and might not cover all cases or be an exact match for the original function.

We can test the function even further to verify that our description is accurate.

COMMAND: PYTHON(./temp/function.py "1a2b3c" "!#@\^_" "zyx" "AaBbCc" "pizza")