{HEAD ~ A first conveying unit} has {TAIL ~ a mechanism} for advancing rollers, which are pressed against the driven portion of an object, toward an intersection, when the object travelling on the first rails nears an intersection, and for retracting the rollers from the driven portion when the object enters the intersection 
{ HEAD ~ A first conveying unit } has { TAIL ~ a mechanism } for advancing rollers , which are pressed against the driven portion of an object , toward an intersection , when the object travelling on the first rails nears an intersection , and for retracting the rollers from the driven portion when the object enters the intersection
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