Find the first occurrence in a string of a substring, starting at 
   a specified location, searching backward. 

   SCANNING 

   VARIABLE  I/O  DESCRIPTION 
   --------  ---  -------------------------------------------------- 
   str        I   Any character string. 
   substr     I   Substring to locate in the character string. 
   start      I   Position to begin looking for substr in str. 

   The function returns the index of the last occurrence of substr in
   str at or preceding index start.

   str        is any character string. 

   substr     is a substring to look for in str.  Spaces in substr are
              significant, including trailing blanks.

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

   The function returns the index of the beginning of the last 
   substring of str that begins at or before index start and is equal 
   to substr. If the substring cannot be found after start, the 
   function is returns -1. 

   None. 

   posr_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 == "AN ANT AND AN ELEPHANT        "
                  012345678901234567890123456789

   Normal (Sequential) Searching: 
   ------------------------------ 

      posr_c ( STRING, "AN",  29 ) == 19
      posr_c ( STRING, "AN",  18 ) == 11 
      posr_c ( STRING, "AN",  10 ) ==  7 
      posr_c ( STRING, "AN",   6 ) ==  3 
      posr_c ( STRING, "AN",   2 ) ==  0 

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

      posr_c ( STRING, "AN", -6 ) == -1
      posr_c ( STRING, "AN", -1 ) == -1 
      posr_c ( STRING, "AN", 30 ) == 19 
      posr_c ( STRING, "AN", 43 ) == 19 

   Significance of Spaces: 
   ----------------------- 

      posr_c ( STRING, "AN",    29 ) ==  19
      posr_c ( STRING, " AN",   29 ) ==  10 
      posr_c ( STRING, " AN ",  29 ) ==  10
      posr_c ( STRING, " AN ",   9 ) ==  -1 
      posr_c ( STRING, " AN  ", 29 ) ==  -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, the search begins at the first 
      character of the string. 

   3) If start is greater than or equal to the length of the string, 
      posr_c returns -1. 

   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, 15-AUG-2002 (NJB) (WLT)

   position of substring reverse search