Consider a push down automata (PDA) below which runs over the input alphabet (a, b). It has the stack alphabet {z0, X} where z0 is the bottom of stack marker. The set of states of PDA is {q0, q1} where q0 is the start state.
The language accepted by PDA is
and in answer
and in answer given that a term and union with that i can not understanding how it can accept it{bn |n>=0}