The function takes a string input, calculates the MD5 hash digest of the input string, and then concatenates the original string with the characters "ln".

[CODE]:
```python
import hashlib

def f(x):
    hashed_string = hashlib.md5(x.encode()).hexdigest()
    transformed_string = x + "ln"
    return transformed_string
```

This updated code incorporates the MD5 hash calculation as well as the string concatenation with "ln" to match the observed behavior of the function.