Insert the character you type.
Whichever character you type to run this command is inserted.
Before insertion, `expand-abbrev' is executed if the inserted character does
not have word syntax and the previous character in the buffer does.
After insertion, the value of `auto-fill-function' is called if the
`auto-fill-chars' table has a non-nil value for the inserted character.
At the end, it runs `post-self-insert-hook'. (fn N)