4. A 2-stack PDA is similar to an ordinary PDA, but has two stacks. Thus, any transition for a 2-stack PDA has the form (q,x,Z1,Z2) = (p,X1,X2), meaning that in state q, looking at input symbol x with Z1 on top of stack 1, and Z2 on top of stack 2, the machine passes to state p, pops Zi and pushes Xi on stack i, where Xi is some stack symbol (or , of course). Show that 2-stack PDAs are more powerful than ordinary PDAs by describing (informally, but clearly) a 2-stack PDA that recognizes the non-CFL {w!w | w {0,1}}.