Cast is related to Bone. Bone is the opposite of Muscle. you are likely to find Muscle in/at/on Leg. Leg is related to Ankle. 
Police is related to Protest. Protest is related to Pro. Pro is etymologically related to Professional. Practitioner is a type of Professional. 
Cast <--RelatedTo--> Bone <--Antonym--> Muscle --AtLocation--> Leg <--RelatedTo--> Ankle 
Police <--RelatedTo--> Protest <--RelatedTo--> Pro --EtymologicallyRelatedTo--> Professional <--IsA-- Practitioner 