reserve fi,psi for Ordinal-Sequence,
  A,A1,B,C,D for Ordinal,
  X,Y for set,
  x,y for object;

theorem Th14:
  A in 1 implies A = {}
proof
  assume A in 1;
  hence A c= {} & {} c= A by Lm1,ORDINAL1:22;
end;
