the BLT extension library for Tcl/Tk - development files 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.