```ts
/** Lay a straight eastward conveyor run across a factory floor plan. */
export function layConveyor(floor: string[], row: number, col: number, length: number): string[] {
  if (row < 0 || row >= floor.length) {
    throw new Error("the run starts off the plan");
  }
  const cells = floor[row].split("");
  if (col + length > cells.length) {
    throw new Error("the run passes the last column");
  }
  for (let i = 0; i < length; ++i) {
    if (cells[col + i] !== ".") {
      throw new Error("a cell in the run is not open");
    }
    cells[col + i] = "=";
  }
  return floor.map((r, rIdx) => (rIdx === row ? cells.join("") : r));
}
```