This is a ring (see section rings) of regions deleted from buffers. Some commands save affected regions on the kill ring before performing modifications. You should consider making the command undoable (see section undo), but this is a simple way of achieving a less satisfactory means for the user to recover.
This kills region saving it in kill-ring. Current-type is either
:kill-forward or :kill-backward. When the last-command-type
is one of these, this adds region to the beginning or end, respectively, of
the top of kill-ring. The result of calling this is undoable using the
command Undo (see the Hemlock User’s Manual). This sets
last-command-type to current-type, and it interacts with
kill-characters.
kill-characters kills count characters after mark if count is
positive, otherwise before mark if count is negative. When count
is greater than or equal to Character Deletion Threshold, the killed
characters are saved on kill-ring. This may be called multiple times
contiguously (that is, without last-command-type being set) to
accumulate an effective count for purposes of comparison with the threshold.
This sets last-command-type, and it interacts with kill-region. When
this adds a new region to kill-ring, it sets last-command-type to
:kill-forward (if count is positive) or :kill-backward (if
count is negative). When last-command-type is :kill-forward or
:kill-backward, this adds the killed characters to the beginning (if
count is negative) or the end (if count is positive) of the top of
kill-ring, and it sets last-command-type as if it added a new region
to kill-ring. When the kill ring is unaffected, this sets
last-command-type to :char-kill-forward or :char-kill-backward
depending on whether count is positive or negative, respectively.
This returns mark if it deletes characters. If there are not count characters in the appropriate direction, this returns nil.