Law is the opposite of Chaos. Chaos is the opposite of Organization. Organization is related to National. 
you are likely to find Person in/at/on Conference. you are likely to find Vendor in/at/on Conference. Vendor is a synonym of Seller. 
Law <--Antonym--> Chaos <--Antonym--> Organization <--RelatedTo--> National 
Person --AtLocation--> Conference <--AtLocation-- Vendor <--Synonym--> Seller 