is a superstructure with:
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(a) enclosing bulkheads of efficient construction;
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(b) access openings, if any, in those bulkheads
fitted with permanently attached weathertight doors of a strength
equivalent to the unpierced structure which can be operated from each
side; and
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(c) other openings in sides or ends of the superstructure
fitted with efficient weathertight means of closing.
A bridge or poop shall not be regarded as enclosed unless
access is provided for the crew to reach machinery and other working
spaces inside those superstructures by alternative means which are
available at all times when bulkhead openings are closed.