The function takes a string input and calculates the sum of the index positions of each character in the string according to the English alphabet, then appends "x" to the result.

[CODE]: 
```python
def f(x):
    alphabet = "abcdefghijklmnopqrstuvwxyz"
    index_sum = sum([alphabet.index(char.lower()) + 1 for char in x])
    result = f"{index_sum}x"
    return result
```

To confirm this, let's test some new inputs:

COMMAND: PYTHON(./temp/function.py "uvwxyz" "programming" "assistant")