
theorem Th61:
for G being SimpleGraph, S being Subset of Vertices G, v being object
 st S = {v} holds S is stable
proof
  let G be SimpleGraph, S be Subset of Vertices G, v be object such that
A1: S = {v};
  let x, y be set such that
A2: x <> y and
A3: x in S & y in S;
   x = v & y = v by A1,A3,TARSKI:def 1;
  hence {x,y} nin G by A2;
end;
