Classify the grammar according to the Chomsky hierarchy

Question

S ->bA|aB A ->a|aS|bAA B ->b|bS|aBB I thought this is the type of context-free grammar type, but I am not sure why it's not unrestricted grammar or context-sensitive grammar. In my opinion, context-free it's a special kind of context-sensitive or unrestricted grammar. I don't know how to explain why it's not the other two types. If someone can help with it, thank you!

Answer 1

The Chomsky hierarchy is a hierarchy, not a partition. It was originally proposed as a series of restrictions, with regular grammars being the most restricted. So each level is a restricted subset of the previous level, and that's how you should think of it

Or you could think of it like the taxonomic hierarchy: a kangaroo is a marsupial, a mammal, a vertebrate and an animal. That's a containment series, in which each level refines the previous one; the kangaroo doesn't stop being a mammal when it's classified as a marsupial.