Find the first occurrence in a string of a character belonging 
   to a collection of characters, starting at a specified location, 
   searching in reverse.

   SCANNING 

   VARIABLE  I/O  DESCRIPTION 
   --------  ---  -------------------------------------------------- 
   str        I   Any character string. 
   chars      I   A collection of characters. 
   start      I   Position to begin looking for one of chars. 

   The function returns the index of the last character of str 
   at or before index start that is in the collection chars. 

   str        is any character string. 

   chars      is a character string containing a collection 
              of characters.  Spaces in chars are significant,
              including trailing blanks.  The order in which
              characters are listed is not significant.

   start      is the position in str to begin looking for one of 
              the characters in chars.  start may range from 0
              to n-1, where n is the number of characters in str.

   The function returns the index of the last character of str (at or
   before index start) that is one of the characters in the string
   chars.  The returned value normally ranges from 0 to n-1, where n is
   the number of characters in str. If none of the characters is found,
   the function returns -1.

   None. 

   cposr_c is case sensitive. 

   An entire family of related CSPICE routines

      cpos_c
      cposr_c
      ncpos_c
      ncposr_c 
      pos_c
      posr_c 

   is described in the Required Reading. 

   Let string == "BOB, JOHN, TED, AND MARTIN...." 
                  012345678901234567890123456789


   Normal (sequential) searching: 
   ------------------------------ 

     cposr_c( string, ' ,',    29 ) = 29 
     cposr_c( string, ' ,',    28 ) = 28 
     cposr_c( string, ' ,',    27 ) = 27 
     cposr_c( string, ' ,',    26 ) = 26 
     cposr_c( string, ' ,',    25 ) = 19 
     cposr_c( string, ' ,',    18 ) = 15 
     cposr_c( string, ' ,',    14 ) = 14 
     cposr_c( string, ' ,',    13 ) = 10 
     cposr_c( string, ' ,',     9 ) =  9 
     cposr_c( string, ' ,',     8 ) =  4 
     cposr_c( string, ' ,',     3 ) =  3 
     cposr_c( string, ' ,',     2 ) = -1 


   start out of bounds: 
   -------------------- 

     cposr_c( string, ' ,',   230 ) = 29 
     cposr_c( string, ' ,',    30 ) = 29 
     cposr_c( string, ' ,',    -1 ) = -1 
     cposr_c( string, ' ,',   -10 ) = -1 

   None. 

   1) The error SPICE(NULLPOINTER) is signaled if either of 
      the input string pointers is null.

   2) If start is less than 0, cposr_c returns -1. 

   3) If start is greater than or equal to the length of the string, 
      the search begins at the last character of the string. 

   4) The function returns -1 if either of the input strings is empty.

   None. 

   N.J. Bachman    (JPL) 
   W.L. Taber      (JPL) 

   None. 

   -CSPICE Version 1.0.0, 27-AUG-2002 (NJB) (WLT)

   backward search for position of character