2PDA · Recursively enumerable
aⁿbⁿcⁿ — beyond one stack
The language that defines the limit of context-free: equal runs of a, b, and c. Stack 1 matches a's to b's while stack 2 matches b's to c's — two stacks reach Turing power.
The author’s examples, run
abc → acceptaabbcc → acceptaaabbbccc → acceptaabbc → rejectabcc → rejectacb → reject