'Add tag NEWTAG to item which is closest to pixel at X, Y.
If several match take the top-most.
All items closer than HALO are considered overlapping (all are
closests). If START is specified the next below this tag is taken.'

def addtag_closest(self, newtag, x, y, halo=None, start=None):

    self.addtag(newtag, 'closest', x, y, halo, start)
