请给出一个上下文无关文化 context free grammar,或者乔姆斯基范式。
Give a context free grammar in Chomsky Normal Form whose language is the set ofstrings on{ a,b,c} which are of odd length, have middle character b, and for which the number ofa’s in the first half of the word is even, and in the second half is odd.
就是一个abc的字符集上，生成的字符串中间必须是b，b左边有偶数个a，右边必须奇数个a。
这个语法我实在是不知道怎么做了。
a-> a
aa -> aa* b
b -> a
ba -> aa*