[[Explain essence.]]

CRC is an acronym for ''Cyclic Redundancy Check'' [http://whatis.techtarget.com/definition/0,,sid9_gci213868,00.html]
[http://www.rad.com/networks/1994/err_con/crc.htm].


















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CRCs are available to Tcl programmers in several forms, including
[Andreas Kupries]' [Trf] package.  [Pat Thoyts] illustrates example
manipulations which rely on this C-coded extension:
      32 % package require Trf
      2.1
      33 % set chk [crc {Hello, World!}]
      <funny chars>
      34 % binary scan $chk h* chkx
      1
      35 % set chkx
      fdb8ec
      37 % binary format h* $chkx
      <funny chars>

Note that the [[hex]] command in Trf can be used to perform the same conversions. Here is the example from above recast to use [[hex]].

      32 % package require Trf
      2.1
      33 % set chk [crc {Hello, World!}]
      <funny chars>
      34 % set chkx [hex -mode encode -- $chk]
      fdb8ec
      37 % hex -mode decode -- $chkx
      <funny chars>
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[[Give reference to pure-Tcl implementation.]]