patch and change/delete.

it's like if i had a book you wrote and you wanted to make some revisions to my copy. you could print the entire book and give it me a new one (redownload the entire 500MB), or you could just say "hey on page 30, remove the second paragraph, please add this image to the index, and on page 36 there is a typo change that one word" (patch just 100MB)