http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/unknown.htm 

The '''unknown''' procedure is called if a command is not known to the interpreter. The default '''unknown''' handler tries to find a sourcable file to define the command in question, or, in interactive mode, tries to redirect the command to the operating system. For examples how to put '''unknown''' to other uses, see [Radical language modification]. 
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As hinted above, manipulation of unknown can be a powerful
[metaprogramming] technique, giving Tcl radically new behavior.
unknown is implemented as a proc; among other reasons, it does
elaborate string-handling that'd be tedious in C.  One might
imagine, though, in a
"[Small Tcl]"
context, construction of a minimal C-coded unknown implementation
that both provides sufficient functionality for embedded situations
(autoloading?) and occupies severely constrained memory.
















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