Commnands:
c <range>: change a range of lines
l <range>: list a range of lines, or the whole file if no range is given.
s <string>: search for <string> in the file
S <string>: case sensitive search
i <linenumber>: insert at <linenumber>
a: append to the end of file
d <range>: delete a range of lines
w: write the file out
q: quit the editor

when you're in insert mode, typing '.' on a line by itself will get
you back to command mode.