The function replaces the lowercase character 'i' with an 'R' and appends the original input string after the replacement.

[CODE]:
```python
def f(x):
    return 'R' + x.replace('i', 'R') + x
```

Please note that this is only an approximation based on the limited information provided. Let me know if there are more experiments you would like me to run or if you have any specific questions about the function.