— Command: yank &optional arg

This command inserts before point the text at the front of the kill ring. It sets the mark at the beginning of that text, using push-mark (see The Mark), and puts point at the end.

If arg is a non-nil list (which occurs interactively when the user types C-u with no digits), then yank inserts the text as described above, but puts point before the yanked text and sets the mark after it.

If arg is a number, then yank inserts the argth most recently killed text—the argth element of the kill ring list, counted cyclically from the front, which is considered the first element for this purpose.

yank does not alter the contents of the kill ring, unless it used text provided by another program, in which case it pushes that text onto the kill ring. However if arg is an integer different from one, it rotates the kill ring to place the yanked string at the front.

yank returns nil.

— Command: yank-pop &optional arg

This command replaces the just-yanked entry from the kill ring with a different entry from the kill ring.

This is allowed only immediately after a yank or another yank-pop. At such a time, the region contains text that was just inserted by yanking. yank-pop deletes that text and inserts in its place a different piece of killed text. It does not add the deleted text to the kill ring, since it is already in the kill ring somewhere. It does however rotate the kill ring to place the newly yanked string at the front.

If arg is nil, then the replacement text is the previous element of the kill ring. If arg is numeric, the replacement is the argth previous kill. If arg is negative, a more recent kill is the replacement.

The sequence of kills in the kill ring wraps around, so that after the oldest one comes the newest one, and before the newest one goes the oldest.

The return value is always nil.

— Variable: yank-undo-function

If this variable is non-nil, the function yank-pop uses its value instead of delete-region to delete the text inserted by the previous yank or yank-pop command. The value must be a function of two arguments, the start and end of the current region.

The function insert-for-yank automatically sets this variable according to the undo element of the yank-handler text property, if there is one.