theorem Th23:
  [!f,x-h/2,x+h/2!] = (cdif(f,h).1.x)/h
proof
  [!f,x-h/2,x+h/2!] = (cD(f,h).x)/h by Th22
    .= (cdif(f,h).1.x)/h by Th16;
  hence thesis;
end;
