i+k = j+l
==> i+k-j = l ===> i - j + k = l
push x's for each a, then pop x's for each b, if stack is empty then push y for each b,
push x's for each c, if top of stack is x,
if top of stack is y, then pop y for each c
if top of stack is empty, then push x for each c
pop x's for each d, if stack empty at final, then accept it !