A macro and/or template processor written in Tcl for Tcl, by William Duquette. See http://www.wjduquette.com/expand/index.html for its homepage.
This is a useful tool for conversion of tcl-based documents into other formats. The range of applications include
All of this falls under the broad category of Template and Macro processing.
Category Application | Category Package, a part of Tcllib (/textutil)