BLT is a library of useful extensions for the Tcl language and the
 popular Tk graphical toolkit. It adds a vector and tree data type,
 background execution and some debugging tools to Tcl, and provides
 several new widgets for Tk, including graphs, bar-charts, trees, tabs,
 splines and hyper-links, as well as a new geometry manager, drag &
 drop support, and more.
 .
 This package contains the headers and libraries needed to extend or
 embed BLT.