WELL the pumping lemma and pushdown automata these two topic are the most hard topic of this semester . And probably the most important topics of the computer science because these topics are the fundamental of CS . all the high level language are based on these theory . And as i know i am not going to understand these topics till the end of the semester (ACTULLY THIS IS THE END OF SEMESTER) . so i would like to thank all those people who have created these mysterious(TO ME ) theory and lemma . well i always try to understand these lemma but not a single time they have enter my head , i always wonder how people like chomsky thought all this for first time in this universe . That’s why i am really thank full to these great people .