theorem Th33:
  H1 is_point_conv_on X & H2 is_point_conv_on X implies for x st x
in X holds H1#x + H2#x = (H1+H2)#x & H1#x - H2#x = (H1-H2)#x & (H1#x) (#) (H2#x
  ) = (H1(#)H2)#x
by Th30;
