theorem Th24:
  the_midpoint_of_the_segment(A,A)=A
  proof
    the_midpoint_of_the_segment(A,A) = 1/2*(A+A) by Th22
                              .= 1/2 * A + 1/2 * A by RVSUM_1:51
                              .= (1/2+1/2)*A by RVSUM_1:50
                              .= A by RVSUM_1:52;
    hence thesis;
  end;
