def _mobius_to_interval(M, field):
 (a, b, c, d) = M
 (s, t) = (field(a, c), field(b, d))
 if (s <= t):
  return (s, t)
 else:
  return (t, s)

def _mobius_to_interval ( M , field ) : ( a , b , c , d ) = M ( s , t ) = ( field ( a , c ) , field ( b , d ) ) if ( s <= t ) : return ( s , t ) else : return ( t , s )
convert a mobius transform to an open interval .