surface -- the surface key of a Divisor or a Curve

i1 : C = curve(5,2);

i2 : D = divisor C

o2 = {2, 3}

o2 : Divisor

i3 : Q = surface D

o3 = ideal(- x x  + x x )
              1 2    0 3

o3 : QuadricSurface