              To transform the vector `srfvec' from a reference frame
               `fixref' at time `trgepc' to a time-dependent reference
               frame `ref' at time `et', the routine <a href="pxfrm2_c.html">pxfrm2_c</a> should be
               called. Let `xform' be the 3x3 matrix representing the
               rotation from the reference frame `fixref' at time
               `trgepc' to the reference frame `ref' at time `et'. Then
               `srfvec' can be transformed to the result `refve