0    1          IF
   1    2          NUMBER
   2    7          expr -> NUMBER
   7    8          THEN
   8    1          IF
   1    2          NUMBER
   2    7          expr -> NUMBER
   7    8          THEN
   8    2          NUMBER
   2    6          expr -> NUMBER
   6    5          expr-stmt -> expr
   5    9          stmt -> expr-stmt
   9   10          ELSE
  10    2          NUMBER
   2    6          expr -> NUMBER
   6    5          expr-stmt -> expr
   5   11          stmt -> expr-stmt
  11    4          if-stmt -> IF expr THEN stmt ELSE >! stmt
   4    9          stmt -> if-stmt
   9    4          if-stmt -> IF expr THEN stmt
   4    3          stmt -> if-stmt
   3               start -> stmt