for GS being GraphStruct
for x, y being set
for n1, n2 being Nat st n1 <> n2 holds
( n1 in dom ((GS .set (n1,x)) .set (n2,y)) & n2 in dom ((GS .set (n1,x)) .set (n2,y)) & ((GS .set (n1,x)) .set (n2,y)) . n1 = x & ((GS .set (n1,x)) .set (n2,y)) . n2 = y )