incremental output please

It would be great to be able to get terminals a grammar is sure about out in a stream fashion.

e.g.
parser = PyMeta..()
parser.completed_callback = lambda x:sys.stdout.write(x + '\n')
parser.parse(stuff)

When used this should be aggressive - once something is determined it should be output - this will permit its use as (e.g.) a parser for protocols.