Trailer is related to Back. Back is related to Pay. Pay is related to Tender. Tender is related to Gentle. 
Closed is a form of Close. Closed is similar to Union. Union is a synonym of Coalition. 
Trailer <--RelatedTo--> Back <--RelatedTo--> Pay <--RelatedTo--> Tender <--RelatedTo--> Gentle 
Close <--FormOf-- Closed <--SimilarTo--> Union <--Synonym--> Coalition 