reserve A,B for Ordinal,
  K,M,N for Cardinal,
  x,x1,x2,y,y1,y2,z,u for object,X,Y,Z,X1,X2, Y1,Y2 for set,
  f,g for Function;
reserve m,n for Nat;
reserve x1,x2,x3,x4,x5,x6,x7,x8 for object;

theorem
  for M1, M2 being set st card M1 = 0 & card M2 = 0 holds M1 = M2
proof
  let M1, M2 be set;
  assume that
A1: card M1 = 0 and
A2: card M2 = 0;
  M1 = {} by A1;
  hence thesis by A2;
end;
