Documentation can be found for various (tcllib) data structures.
Provided structures, so far:

   * [tree] - http://tcllib.sourceforge.net/doc/tree.html
   * [graph] - http://tcllib.sourceforge.net/doc/graph.html
   * [queue] - http://tcllib.sourceforge.net/doc/queue.html
   * [stack] - http://tcllib.sourceforge.net/doc/stack.html
   * [matrix] - http://tcllib.sourceforge.net/doc/matrix.html

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Often what people want from "struct" is a C-like datatype.  This is much
different from the tcllib data structures described above.  Various people have coded Tcl extensions for this kind of "struct";
[Iain B. Findleton]'s [Containers] appears to be the most current.
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[Category Package], subset [Tcllib]