unread-char places character back onto the front of
input-stream so that it will again be the next character
When input-stream is an echo stream,
no attempt is made to undo any echoing of the character that might already
have been done on input-stream. However, characters placed on
input-stream by unread-char are marked in such a way
as to inhibit later re-echo by read-char.
It is an error to invoke unread-char
twice consecutively on the same stream
without an intervening call to read-char
(or some other input operation which implicitly reads characters)
on that stream.
Invoking peek-char or read-char commits all previous characters.
The consequences of invoking unread-char
on any character preceding that which is returned by
peek-char (including those passed over by
peek-char that has a non-nil peek-type)
In particular, the consequences of
invoking unread-char after peek-char