A worker cleans a stairway under the supervision of his co-worker .
A worker is mopping a stairway .
A worker cleans a stairway under the supervision of his co-worker.
A worker is mopping a stairway.
Clearly he's a worker on a stairway, but mopping would be out of the question here.
Clearly he 's a worker on a stairway , but mopping would be out of the question here .