```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) or not all(cell == "." for cell in cells[col:col+length]):
        raise ValueError("the run would leave the plan or cover non-open cells")
    for i in range(length):
        cells[col + i] = "="
    return floor
```