Example 6.2.1.The language L1 = {akbkck | k 0} over alphabet {a, b, c}is not context-free.
Example 6.2.2.The language L2 = {apbqcpdq | p,q 0} over alphabet {a, b, c, d} is not context-free.
Note however, that the language {apbqcqdp | p,q 0} IS context-free.
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