'Returns a new StringIOTree, which is left behind at the current position
(it what is written to the result will appear right before whatever is
next written to self).
Calling getvalue() or copyto() on the result will only return the
contents written to it.'

def insertion_point(self):

    self.commit()
    other = StringIOTree()
    self.prepended_children.append(other)
    return other
