yes number of b in x , and number of a in y could be different
example bbb could be a string with only b in x
bbaaa - bb is part that b in x and aaa is part that a in y
bbabbb - here say bbabb in x and b in y [here we have to maintain a restriction, we cannot take any thing as x and anything as y]
b3 an bn-1 - here b3an-1 in x part and abn-1 in y part. right?
though it is not making any problem in final string getting (a+b)*