sentence1: a context-free grammar ( cfg ) is a 4-tuple math-w-3-1-1-9 where math-w-3-1-1-21 and math-w-3-1-1-23 are finite disjoint sets of nonterminal and terminal symbols , respectively , math-w-3-1-1-36 is the start symbol and math-w-3-1-1-44 is a finite set of rules .
sentence2: a context-free grammar ( cfg ) is a tuple math-w-2-5-5-22 , where vn and vt are finite , disjoint sets of nonterminal and terminal symbols , respectively , and s e vn is the start symbol .