TES is David Gravereaux's Windows-specific Tcl-interpreter-in-C++-application embedding experiment.
He maintains it at http://sourceforge.net/projects/tomasoft/ .
The acronym stands for Tcl Event System.
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In a nutshell, TES is a way to separate the Tcl event loop from interactions with the Windows message pump of the application Tcl is embedded into. It does this by running Tcl in a separate thread and provides the way to post Tcl jobs. This is a perfect solution for running Tk, which requires an active event loop. It avoids the need to merge event loops or use flushing by doing a sledgehammer approach instead.
Or another way to look at, TES allows for modular execution of Tcl, the component, which might be comparable to Microsoft's COM.