I have a simple question. A student in computer science asked me the question. This is not her homework (This is the starting semester point in our country!). 
 If $W_1W_2 | W_1,W_2 \in (a+b)^* , N_a(W_1)=N_b(W_2)$ is context free? If NPDA can construct the language? There is a book here that claimed this is not CF (without any description :( ), but I think we can construct a NPDA to build the language.