(rule-)head results (rule-)bodystart symbol should have non-terminal or empty stringnonterminal (head)→{terminal ∣ nonterminal ∣ empty} (body)nonterminal \,\,(head) \to \{terminal\, |\,\, nonterminal \,\,|\,\, empty\} \,\,(body)nonterminal(head)→{terminal∣nonterminal∣empty}(body)- notational convention
