| 主权项 |
1. A set of ventilation units comprising first and second ventilation units, wherein a surface of the first ventilation unit is provided with at least one formation and a surface of the second ventilation unit is provided with at least one co-operating formation, wherein said at least one formation and said at least one co-operating formation are engageable with one another in a direction that is substantially perpendicular to said surface of the first ventilation unit and/or to said surface of the second ventilation unit, and wherein the first and second units are arranged to provide between said surfaces of the first and second units, when the at least one formation and the at least one co-operating formation are engaged, an intervening space for receiving a layer of cement. |
