▁this ▁way . ▁Each ▁command ▁implementation ▁function ▁is ▁passed ▁an ▁array ▁of ▁values ▁that ▁describe ▁the ▁( al ready ▁subst it uted ) ▁arguments ▁to ▁the ▁command , ▁and ▁is ▁free ▁to ▁interpret ▁those ▁values ▁as ▁it ▁sees ▁fit . ▁ ▁Digital ▁logic ▁sim ul ators ▁often ▁include ▁a ▁T cl ▁script ing ▁interface ▁for ▁sim ulating ▁Ver ilog , ▁V H DL ▁and ▁System Ver ilog ▁hardware ▁languages . ▁ ▁Tools ▁exist ▁( e . g . ▁SW IG , ▁F f id l ) ▁to ▁automatically ▁generate ▁the ▁necessary ▁code ▁to ▁connect ▁arbitrary ▁C ▁functions ▁and ▁the ▁T cl ▁runtime , ▁and ▁Crit cl ▁does ▁the ▁reverse , ▁allowing ▁embedding ▁of ▁arbitrary ▁C ▁code ▁inside ▁a ▁T cl ▁script ▁and ▁compiling ▁it ▁at ▁runtime ▁into ▁a ▁DLL . ▁ ▁Ext ension ▁packages ▁ ▁The ▁T cl ▁language ▁has ▁always ▁allowed ▁for ▁extension ▁packages , ▁which ▁provide ▁additional ▁functionality , ▁such ▁as ▁a ▁GUI , ▁terminal - based ▁application ▁autom ation , ▁database ▁access , ▁and ▁so ▁on . ▁Comm only ▁used ▁extensions ▁include : ▁▁ ▁T k ▁The ▁most ▁popular ▁T cl ▁extension ▁is ▁the ▁T k ▁tool kit , ▁which ▁provides ▁a ▁graph ical ▁user ▁interface ▁library ▁for ▁a ▁variety ▁of ▁operating ▁systems . ▁Each ▁GUI ▁consists ▁of ▁one ▁or ▁more ▁frames . ▁Each ▁frame ▁has ▁a ▁layout ▁manager . ▁ ▁Ex pect ▁One ▁of ▁the ▁other ▁very ▁popular ▁T cl ▁extensions ▁is ▁Ex pect ▁extension . ▁The ▁early ▁close ▁relationship ▁of ▁Ex pect ▁with ▁T cl ▁is ▁largely ▁responsible