reserve i for Nat,
  j for Element of NAT,
  X,Y,x,y,z for set;

theorem Th8:
  varcl {} = {}
proof
A1: for x,y st [x,y] in {} holds x c= {};
  for B being set st {} c= B & for x,y st [x,y] in B holds x c= B holds {}
  c= B;
  hence thesis by A1,Def1;
end;
