(self)
return (self.xmin, self.xmax)
@return:  (xmin, xmax) tuple for a given tier.
def min_max(self):
    """
        @return:  (xmin, xmax) tuple for a given tier.
        """

    return (self.xmin, self.xmax)
def min_max ( self ) : return ( self . xmin , self . xmax )