'Insert :param:`item` at position :param:`index` in the `Spine`.'

def insert(self, index, item, linear):

    item.linear = self._linear(linear)
    item.spine_position = index
    self.items.insert(index, item)
    for i in xrange(index, len(self.items)):
        self.items[i].spine_position = i
    return item
