Wallcot, a retail chain in US and Canada, wants to improve the security of their administration
offices. They want to implement a mechanism with two doors. Only one of the doors can be opened at a
time. Once people enter from the first door, they have to be authorized to open the next one. Failing
the authorization, the person will be locked between the doors until an authorized person lets him or
her out. What is such a mechanism called?
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