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

theorem
  A in B implies B-^A <> {} & {} in B-^A
proof
  assume A in B;
  then A-^A in B-^A by Th53;
  hence thesis by Th54;
end;
