Chomsky Hierarchy
			Four classes of grammatical formalisms: l Recursively enumerable grammars l Unrestricted rules: both sides of the rewrite rules can have any number of terminal and nonterminal symbols    AB  C l Context-sensitive grammars l The RHS must contain at least as many symbols as the LHS    ASB  AXB l Context-free grammars (CFG) l LHS is a single nonterminal symbol    S  XYa l Regular grammars    X  a       X  aY