def _mobius_from_interval(I, field):
 (s, t) = I
 (a, c) = (field.numer(s), field.denom(s))
 (b, d) = (field.numer(t), field.denom(t))
 return (a, b, c, d)

def _mobius_from_interval ( I , field ) : ( s , t ) = I ( a , c ) = ( field . numer ( s ) , field . denom ( s ) ) ( b , d ) = ( field . numer ( t ) , field . denom ( t ) ) return ( a , b , c , d )
convert an open interval to a mobius transform .