SL -> S SL
   ->
S  -> Id Gets E {3}
   -> Read Id
   -> Write E {5}
E  -> T {7} TT
T  -> F {11} FT
TT -> AO T TT
   ->
FT -> MO F FT
   ->
AO -> Plus
   -> Minus
MO -> Times
   -> DivBy
F  -> Id {13}
   -> Num {17}
   -> LParen E RParen {19}
