```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 would pass the last column");
  }
  for (let i = 0; i < length; i++) {
    if (cells[col + i] !== ".") {
      throw new Error("any cell the run covers is not open");
    }
  }
  const newRow = floor[row].slice(0, col) + "=".repeat(length) + floor[row].slice(col + length);
  return [newRow];
}
```