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   tp_crc32() - compute Zmodem-style CRC

   nCRC := tp_crc32(cString)

   cString     String to checksum.

   Returns     32-bit numeric CRC.

   Computes a 32-bit cyclic redundancy checksum (CRC) by the same
   algorithm used in Zmodem (with 32-bit CRCs enabled).  The value
   returned is numeric, and should be converted to a string with l2bin()
   before you try to send it as data.

   If you append the CRC (after l2bin()) to the end of a string, then
   compute the CRC of the resulting string, you will get the magic
   number 558161692 if the CRC is correct.  That is,

       tp_crc32(string + l2bin(tp_crc32(string))) == 558161692

   [5.0] The header file telepath.ch defines a manifest constant
   TP_32MAGIC as 558161692.