'Write a collapsing line after a node was added at index.'

def _collapse_line(self, index):

    self._indent()
    for c in range(index):
        self._write_edge('|    ', c)
    for c in range(index, len(self._frontier)):
        self._write_edge('    /', c)
    self._set_state(COLLAPSE, index)
    self._out.write('\n')
