can-insert? in text%
Called before items are inserted into the editor.
If the return value is #f, then the
insert will be aborted.
The editor is internally locked for writing during a call to this method
(see also Locks). Use
after-insert to modify the editor, if necessary.
See also
on-insert,
after-insert, and
on-edit-sequence.
( -> boolean
send a-text can-insert? start len)
start : exact non-negative integer
len : exact non-negative integer
The start argument specifies the position of the potential
insert. The len argument specifies the total length (in
positions) of the items to be inserted.