▁a ▁tim ed ▁automat on ▁or ▁a ▁signal ▁automat on , ▁which ▁want ▁to ▁know ▁at ▁each ▁instant ▁whether ▁S ▁holds ▁or ▁not . ▁This ▁system ▁should ▁recall ▁everything ▁that ▁occurred ▁in ▁the ▁last ▁ 1 0 ▁time ▁units . ▁As ▁seen ▁above , ▁it ▁means ▁that ▁it ▁must ▁recall ▁an ▁arbitr arily ▁large ▁number ▁of ▁events . ▁This ▁can ▁not ▁be ▁implemented ▁by ▁a ▁system ▁with ▁finite ▁memory ▁and ▁clock s . ▁ ▁B ounded ▁vari ability ▁ ▁One ▁of ▁the ▁main ▁advantage ▁of ▁M IT L ▁is ▁that ▁each ▁operator ▁has ▁the ▁bounded ▁vari ability ▁property . ▁Example : ▁ ▁Given ▁the ▁statement ▁T ▁defined ▁above . ▁Each ▁time ▁the ▁truth ▁value ▁of ▁T ▁switch es ▁from ▁false ▁to ▁true , ▁it ▁remains ▁true ▁for ▁at ▁least ▁one ▁time ▁unit . ▁Proof : ▁At ▁a ▁time ▁t ▁where ▁T ▁becomes ▁true , ▁it ▁means ▁that : ▁▁ ▁between ▁ 9 ▁and ▁ 1 0 ▁time ▁units ▁ago , ▁P ▁was ▁true . ▁ ▁just ▁before ▁time ▁t , ▁P ▁was ▁false . ▁ ▁Hence , ▁P ▁was ▁true ▁exactly ▁ 9 ▁time ▁units ▁ago . ▁It ▁follows ▁that , ▁for ▁each ▁, ▁at ▁time ▁, ▁P ▁was ▁true ▁ ▁time ▁units ▁ago . ▁Since ▁, ▁at ▁time ▁, ▁T ▁holds . ▁ ▁A ▁system , ▁at ▁each ▁instant , ▁wants ▁to ▁know ▁the ▁value ▁of ▁T . ▁Such ▁a ▁system ▁must ▁recall ▁what ▁occurred ▁during ▁the ▁last ▁ten ▁time ▁units . ▁However , ▁thanks ▁to ▁the ▁bounded ▁vari ability ▁property ,