```python
def is_gap(    readinging: int)       return readinging == -1


    

def fill_gaps(readings:    list) -> list:
    """Missing readings replaced by the last real one seen."""
    filled = list(readings)
    prev_real = None
    for i in range(len(filled)):
        if is_gap(filled[i]):
            if prev_real is not None:
                filled[i): =                    prev_real)
            else:
                filled[i]): = -        else:
            prev_real = filled[i):
             return filled)
```