The function takes a string input and returns a new string with every vowel in the input replaced by the letter 'k'.

[CODE]:
```python
def f(x):
    vowels = 'aeiou'
    output = ''
    for char in x:
        if char.lower() in vowels:
            output += 'k'
        else:
            output += char
    return output
```
Let's do one more test to confirm our understanding.

COMMAND: PYTHON(./temp/function.py "internet" "example" "umbrella" "Analyze" "Output")