```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(floor[0]):
        raise ValueError("the run would pass the last column")
    if any(cell != '.' for cell in cells[col:col + length]):
        raise ValueError("the run would cover a cell that is not open")
    floor[row] = ''.join(['' if cell == '.' else ' conveyor' for cell in cells[col:col + length]])
    return floor
```