'Subtract labels.'

def __sub__(self, other):

    if isinstance(other, Label):
        if (other.hemi == 'lh'):
            lh = (self.lh - other)
            rh = self.rh
        else:
            rh = (self.rh - other)
            lh = self.lh
    elif isinstance(other, BiHemiLabel):
        lh = (self.lh - other.lh)
        rh = (self.rh - other.rh)
    else:
        raise TypeError(('Need:    Label    or    BiHemiLabel.    Got:    %r' % other))
    if (len(lh.vertices) == 0):
        return rh
    elif (len(rh.vertices) == 0):
        return lh
    else:
        name = ('%s    -    %s' % (self.name, other.name))
        return BiHemiLabel(lh, rh, name, self.color)
