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 {HEAD ~ the platform} has moved away from {TAIL ~ the doorway} sill by operation of the parallelogram linkage 
moved away from
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 { HEAD ~ the platform } has moved away from { TAIL ~ the doorway } sill by operation of the parallelogram linkage
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH