```python
"""Lay a straight eastward conveyor run across a factory floor plan."""


def lay_conveyor(floor, row, col, length):
    if row < 0 or row >= len(floor):
        raise ValueError("the run starts off the plan")
    cells = list(floor[row])
    if col + length > len(cells):
        raise ValueError("the run would pass the last column")
    for i in range(length):
        if cells[col + i] != ".":
            raise ValueError("any cell the run covers is not open")
    for i in range(length):
        cells[col + i] = "="
    return floor
```