Andreas Kupries contributed a mechanism that allows I/O channel processing modules to be stacked onto open I/O channels. This adds a new Tcl_StackChannel C API, but there are no changes visible at the Tcl script level. However, it enables several interesting extentions such as compression and encyption (e.g., SSL) to be added to Tcl. Andreas has an extension that exports the channel filter mechanism to the Tcl script level. This is used primarily for testing, but you can also use it for script-level data filters.