'Insert a new :class:`AnchorPoint` at `index`.
See also
NodeAnchorItem.addAnchor'

def insertAnchor(self, index, anchor, position=0.5):

    if (anchor in self.__points):
        raise ValueError(('%s    already    added.' % anchor))
    self.__points.insert(index, anchor)
    self.__pointPositions.insert(index, position)
    anchor.setParentItem(self)
    anchor.setPos(self.__anchorPath.pointAtPercent(position))
    anchor.destroyed.connect(self.__onAnchorDestroyed)
    self.__updatePositions()
    self.setAnchored(bool(self.__points))
    return index
