Difficult is related to Level. Level is related to Boss. Boss is a synonym of Mate. 
the last thing you do when you Travel is Stop. Travel requires Careful. one thing you do when you Driving is Careful. 
Difficult <--RelatedTo--> Level <--RelatedTo--> Boss <--Synonym--> Mate 
Stop <--HasLastSubevent-- Travel --HasPrerequisite--> Careful <--HasSubevent-- Driving 