Return the state transformation matrix from one frame to 
   another at a specified epoch. 

   None. 

   VARIABLE  I/O  DESCRIPTION 
   --------  ---  -------------------------------------------------- 
   from       I   Name of the frame to transform from.
   to         I   Name of the frame to transform to.
   et         I   Epoch of the state transformation matrix.
   xform      O   A state transformation matrix.

   from        is the name of a reference frame in which a state is
               known.

   to          is the name of a reference frame in which it is desired
               to represent the state.

   et          is the epoch in ephemeris seconds past the epoch of
               J2000 (TDB) at which the state transformation matrix
               should be evaluated.

   xform       is the matrix that transforms states from the reference
               frame `from' to the frame `to' at epoch `et'. If (x, y,
               z, dx, dy, dz) is a state relative to the frame `from'
               then the vector ( x', y', z', dx', dy', dz' ) is the
               same state relative to the frame `to' at epoch `et'.
               Here the vector ( x', y', z', dx', dy', dz' ) is defined
               by the equation:

                  -   -       -          -     -  - 
                 | x'  |     |            |   | x  | 
                 | y'  |     |            |   | y  | 
                 | z'  |  =  |   xform    |   | z  | 
                 | dx' |     |            |   | dx | 
                 | dy' |     |            |   | dy | 
                 | dz' |     |            |   | dz | 
                  -   -       -          -     -  - 

   None. 

   This routine provides the user level interface for computing 
   state transformations from one reference frame to another. 

   Note that the reference frames may be inertial or non-inertial. 
   However, the user must take care that sufficient SPICE kernel 
   information is loaded to provide a complete state transformation 
   path from the `from' frame to the `to' frame. 

   Suppose that you have geodetic coordinates of a station on 
   the surface of the earth and that you need the inertial 
   (J2000) state of this station.  The following code fragment 
   illustrates how to transform the position of the station to 
   a J2000 state. 

      #include "SpiceUsr.h"
          .
          .
          .
      bodvcd_c ( 399, radii, 3, &n, abc  ); 

      equatr   =  abc[0]; 
      polar    =  abc[2]; 
      f        = (equatr - polar) / equatr; 

      georec_c ( long, lat, 0.0,  equatr, f,  estate ); 

      estate[3] = 0.0; 
      estate[4] = 0.0; 
      estate[5] = 0.0; 

      sxform_c ( "IAU_EARTH", "J2000",   et,    xform  ); 
      mxvg_c   (  xform,       estate,   6,  6, jstate ); 

   The state `jstate' is the desired J2000 state of the station. 

   None. 

   1) If sufficient information has not been supplied via loaded 
      SPICE kernels to compute the transformation between the 
      two frames, the error will be diagnosed by a routine 
      in the call tree of this routine. 

   2) If either frame `from' or `to' is not recognized the error 
      SPICE(UNKNOWNFRAME) will be signaled. 

   None. 

   C.H. Acton      (JPL)
   N.J. Bachman    (JPL)
   W.L. Taber      (JPL) 

    None. 

   -CSPICE Version 1.1.2, 24-OCT-2005 (NJB)

       Header updates: example had invalid flattening factor
       computation; this was corrected.  Reference to bodvar_c was
       replaced with reference to bodvcd_c.

   -CSPICE Version 1.1.1, 03-JUL-2003 (NJB) (CHA)

       Various header corrections were made.

   -CSPICE Version 1.1.0, 08-FEB-1998 (NJB)  

       References to C2F_CreateStr_Sig were removed; code was
       cleaned up accordingly.  String checks are now done using
       the macro CHKFSTR.

   -CSPICE Version 1.0.0, 25-OCT-1997 (NJB)

      Based on SPICELIB Version 1.0.0, 19-SEP-1995 (WLT)

   Find a state transformation matrix