Excellent! I was waiting for somebody to suggest the informal definition. It has worked for two or three generations of singers, and it beats Tweedle dum and Tweedle dee beating each other over the head with their conference papers. Here's to an informal subjectivity. I wish more people would accept that there is no burden of proof upon the singer, folklorist or writer. Chuck Berry was asked did he play 'Black music or Blues. He answered by saying he hadn't a clue what colour his music was. The simplest answers are often the best.