13.构造产生如下语言的上下文无关文法各一个: (1) (an bm c2m | n,m≥0 } S->AB A->ε|aA B->ε|bBcc (2) w c wR| w∈{a,b}* } S->aSa S->bSb S->c (3) {am bn ck | m=n或n=k } S->aSb|bSc|ε 学如逆水行舟,不进则退