Difficulty is a type of Quality. Quality is related to Success. Success requires Effort. Effort is a type of Labor. 
Processing is related to Product. Product is related to Consumer. Consumer is related to Price. Price is related to Valuable. 
Difficulty --IsA--> Quality <--RelatedTo--> Success --HasPrerequisite--> Effort --IsA--> Labor 
Processing <--RelatedTo--> Product <--RelatedTo--> Consumer <--RelatedTo--> Price <--RelatedTo--> Valuable 