When I was in High School (about 1960) I remember a civics teacher talking about customary enforcement of laws. According to him, if a law is customarily ignored, the authorities cannot begin enforcing it without first giving notice and if it is enforced in one way, it cannot be enforced in a different way such that a person knowing the usual procedure would be charged with a violation because of the change unless there is notice of the change. I have been trying to find legal references to support these claims without success. I have access to a very small law library and full access to Lexis-Nexis, but I have only used it where I had a cite to start with. Where should I start looking?