[web2py] intercepting SQLFORM delete

[Richard]

I want to do some preprocessing before permitting SQLFORM to delete a
record:

topic = db.topic[topic_id]
form = SQLFORM(db.topic, topic)
if form.accepts(request.vars, session, dbio=False):
    if form.vars.get('delete_this_record'):
        response.flash = 'Delete intercepted'
    else:
        response.flash = 'Update intercepted'

However I found dbio only applies to insert/update and doesn't affect
delete (sqlhtml.py: 952, 1031).
Why doesn't dbio apply to all databases operations?
Is there an alternative way to intercept SQLFORM deletion?

thanks,
Richard