Duty is derived from Due. Duty is related to Assignment. 
Work is related to Payment. Payment is related to Owe. Mortgage is a type of Owe. Bond is a type of Mortgage. 
Due <--DerivedFrom-- Duty <--RelatedTo--> Assignment 
Work <--RelatedTo--> Payment <--RelatedTo--> Owe <--IsA-- Mortgage <--IsA-- Bond 