Called just before a top-level (i.e., unnested) edit sequence has started.
During an edit sequence, all callbacks methods are invoked normally, but it may be appropriate for these callbacks to delay computation during an edit sequence. The callbacks must manage this delay manually. Thus, when overriding other callback methods, such as on-insert or after-insert, consider overriding on-edit-sequence and after-edit-sequence as well.
Dispatches to the mode.