def add_child_ff_from_exchange(self, ff, exchange, **kwargs):
"""
Uses a process intermediate exchange to define a child flow to the process. If the exchange's termination
is non-null, then the child exchange will also be terminated.
Want to use a background flow if one exists already
:param ff:
:param exchange:
:return:
"""
try:
bg = next(f for f in self.fragments(background=True) if f.term.terminates(exchange))
f = LcFragment.new(exchange.flow['Name'], exchange.flow, exchange.direction,
parent=ff, exchange_value=exchange.value, **kwargs)
f.terminate(bg)
except StopIteration:
f = LcFragment.from_exchange(ff, exchange)
self.add_entity_and_children(f)
return f
