remove a definite at a section
add an explanation for examples
fix Stack to Queue
an explanation of SimpleStack
add an implementation of SimpleStack and the lemma; write how to prove it
fix a bug in SimpleStack
add .gitignore
fix a bit at the introduction
may not published about SimpleStack
may not published about GCD
add an example of addtion
add an explanation of verification for some propaties of addition
fix URLs of images
commit images