'A list of all left siblings of this tree, in any of its parent
trees.  A tree may be its own left sibling if it is used as
multiple contiguous children of the same parent.  A tree may
appear multiple times in this list if it is the left sibling
of this tree with respect to multiple parents.
:type: list(MultiParentedTree)'

def left_siblings(self):

    return [parent[(index - 1)] for (parent, index) in self._get_parent_indices() if (index > 0)]
