reserve a,b,c,d for Ordinal;
reserve l for non empty limit_ordinal Ordinal;
reserve u for Element of l;
reserve A for non empty Ordinal;
reserve e for Element of A;
reserve X,Y,x,y,z for set;
reserve n,m for Nat;

theorem Th4:
  a c= b iff b nin a
  proof
    a c= b & b in a implies b in b;
    hence thesis by ORDINAL1:16;
  end;
