The function takes an input string, reverses it, and inserts the character '7' after the third character of the reversed string.

[CODE]:
```
def approx_function(x):
    reversed_x = x[::-1]
    return reversed_x[:3] + '7' + reversed_x[3:]
```

I'll do a few more tests to confirm the behavior.

COMMAND: PYTHON(./temp/function.py "reverse" "string" "operation" "insert" "number")