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.
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.