Tcl is a highly extensible and introspective language. The core commands are progressively being replaced by ensembles, and this means that one can add new sub-commands to any of them, wrap any extant subcommand with new functionality (and thereby completely change, or simply augment its functionality.) Another useful skinning technique is RS's know command, enabling you to create new commands by matching their names with regexps on-the-fly. Skins should be composable - so that as long as there's no name-collision, multiple skins from different authors can co-exist.
The process of extending existing core commands is hereby and hereinafter called skinning, and there is more than one way to skin a language. The only caveat is: skins need to be packaged so that they can be package required.
Some candidates for skinning are:
* [info] * [dict] * [oo::class] * [package]