The doorway barriers are movably mounted to the posts of the fixed frame in the doorway to block passage by protruding into the doorway opening when the platform has moved away from the doorway sill by {HEAD ~ operation} of {TAIL ~ the parallelogram linkage} 
The doorway barriers are movably mounted to the posts of the fixed frame in the doorway to block passage by protruding into the doorway opening when the platform has moved away from the doorway sill by { HEAD ~ operation } of { TAIL ~ the parallelogram linkage }
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