---
sum := sum plus sum
  sum := sum plus sum
    sum := "INT"
      INT "11" (1, 1)
    plus := "PLUS"
      PLUS "+" (1, 4)
    sum := "INT"
      INT "22" (1, 6)
  plus := "PLUS"
    PLUS "+" (1, 9)
  sum := "INT"
    INT "33" (1, 11)