              frame `from' at epoch `etfrom' to the frame `to' at epoch
              `etto'.

              If (x, y, z) is a position relative to the reference
              frame `from' at time `etfrom' then the vector ( x', y',
              z') is the same position relative to the frame `to' at
              epoch `etto'. Here the vector ( x', y', z' ) is defined
              by the equation:

                 -   -       -        -     -  -
                | x'  |     |          |   | x  |
          