Gasoline is related to Pump. Pump is related to Water. Water is related to Current. Current is related to Modern. 
Visitor is a type of Traveler. Guest is a type of Visitor. 
Gasoline <--RelatedTo--> Pump <--RelatedTo--> Water <--RelatedTo--> Current <--RelatedTo--> Modern 
Traveler <--IsA-- Visitor <--IsA-- Guest 