I have problem in understanding following eq of Push down automata from the book:

L= {a^n b^n: n>=0} U {a}

I am able to understand, when we push 'a', then '1' would be stored in the stack. And when we pop 'a' from the stack then 1' is removed from the stack.

This helps to keep track of the fact that count of 'a' and 'b' is equal.

But why the above equation only show addition operation and not the removal operation.

Zulfi.