theorem Th5:
  (FuncAdd(X,Y)).(FuncZero(X,Y),f) = f
proof
  now
    let x be Element of X;
    thus ((FuncAdd(X,Y)).(FuncZero(X,Y),f)).x = (FuncZero(X,Y)).x + f.x by
LOPBAN_1:1
      .= 0.Y + f.x by Th1
      .= f.x by RLVECT_1:def 4;
  end;
  hence thesis by FUNCT_2:63;
end;
