Merchant is related to Occupation. Occupation is a synonym of Business. Business is related to Economy. Economist is derived from Economy. 
Oxygen is related to Essential. Essential is related to Requirement. Requirement is related to Duty. Duty is related to Service. 
Merchant <--RelatedTo--> Occupation <--Synonym--> Business <--RelatedTo--> Economy <--DerivedFrom-- Economist 
Oxygen <--RelatedTo--> Essential <--RelatedTo--> Requirement <--RelatedTo--> Duty <--RelatedTo--> Service 