An engagement occurs between {HEAD ~ the first and second elements} in {TAIL ~ the closed condition} of the abutment structure 
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An engagement occurs between { HEAD ~ the first and second elements } in { TAIL ~ the closed condition } of the abutment structure
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH