Assert the internal consistency of the instance's data structures.
        This method is for debugging only.

def _check(self):
        """
        Assert the internal consistency of the instance's data structures.
        This method is for debugging only.
        """
        for k,ix in six.iteritems(self._indices):
            assert k is not None, 'null key'
            assert ix, 'Key does not map to any indices'
            assert ix == sorted(ix), "Key's indices are not in order"
            for i in ix:
                assert i in self._lines, 'Key index does not map to line'
                assert self._lines[i].key is not None, 'Key maps to comment'
                assert self._lines[i].key == k, 'Key does not map to itself'
                assert self._lines[i].value is not None, 'Key has null value'
        prev = None
        for i, line in six.iteritems(self._lines):
            assert prev is None or prev < i, 'Line indices out of order'
            prev = i
            if line.key is None:
                assert line.value is None, 'Comment/blank has value'
                assert line.source is not None, 'Comment source not stored'
                assert loads(line.source) == {}, 'Comment source is not comment'
            else:
                assert line.value is not None, 'Key has null value'
                if line.source is not None:
                    assert loads(line.source) == {line.key: line.value}, \
                        'Key source does not deserialize to itself'
                assert line.key in self._indices, 'Key is missing from map'
                assert i in self._indices[line.key], \
                    'Key does not map to itself'