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
  x in a\b iff b c= x & x in a
  proof
    x in a\b iff x nin b & x in a by XBOOLE_0:def 5;
    hence thesis by Th4;
  end;
