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

theorem Th56:
  A-^{} = A & {}-^A = {}
proof
A1: {}+^A = A by ORDINAL2:30;
  {} c= A;
  hence A-^{} = A by A1,Def5;
  not A c= {} or A c= {};
  then thesis or A = {} by Def5,XBOOLE_1:3;
  hence thesis by A1,Def5;
end;
