▁the ▁current ▁procedure ▁to ▁refer ▁to ▁variables ▁in ▁an ▁en clos ing ▁procedure ▁call ▁or ▁to ▁global ▁variables . ▁The ▁up var ▁command ▁simpl ifies ▁the ▁implementation ▁of ▁call - by - name ▁procedure ▁calling ▁and ▁also ▁makes ▁it ▁easier ▁to ▁build ▁new ▁control ▁construct s ▁as ▁T cl ▁procedures . ▁ ▁A ▁dec r ▁command ▁that ▁works ▁like ▁the ▁built - in ▁inc r ▁command ▁except ▁it ▁subtract s ▁the ▁value ▁from ▁the ▁variable ▁instead ▁of ▁adding ▁it : ▁▁ ▁proc ▁dec r ▁{ var Name ▁{ de crement ▁ 1 }} ▁{ ▁▁▁▁ ▁up var ▁ 1 ▁$ var Name ▁var ▁▁▁▁ ▁inc r ▁var ▁[ expr ▁{ -$ de crement }] ▁} ▁ ▁Object - orient ed ▁▁ ▁T cl ▁ 8 . 6 ▁added ▁a ▁built - in ▁dynamic ▁object ▁system , ▁T cl O O , ▁in ▁ 2 0 1 2 . ▁It ▁includes ▁features ▁such ▁as : ▁ ▁Class - based ▁object ▁system . ▁This ▁is ▁what ▁most ▁program mers ▁expect ▁from ▁O O . ▁ ▁All ows ▁per - object ▁custom ization ▁and ▁dynamic ▁re definition ▁of ▁classes . ▁ ▁Meta - classes ▁ ▁Fil ters ▁ ▁Mix ins ▁ ▁A ▁system ▁for ▁implementing ▁methods ▁in ▁custom ▁ways , ▁so ▁that ▁package ▁authors ▁that ▁want ▁significantly ▁different ▁ways ▁of ▁doing ▁a ▁method ▁implementation ▁may ▁do ▁so ▁fairly ▁simply . ▁ ▁o o :: class ▁create ▁fruit ▁{ ▁▁▁▁ ▁method ▁eat ▁{} ▁{ ▁▁▁▁▁▁▁▁ ▁puts ▁" y ummy !" ▁▁▁▁ ▁} ▁} ▁o o :: class