TRACE OF RECURSIVE-DESCENT PARSER FOR PL/0 ATTRIBUTE GRAMMAR
                ------------------------------------------------------------

        vax X, Y;
        begin
          X := 0;
          Y := X + 1
        end.

        PROGRAM (out TREE)
          GET_TOKEN (TOKEN = var)
          BLOCK (out ENV, TREE)
            GET_TOKEN (TOKEN = X)
            GET_TOKEN (TOKEN = ,)
            GET_TOKEN (TOKEN = Y)
            GET_TOKEN (TOKEN = ;)
            GET_TOKEN (TOKEN = begin)
            STATEMENT (in ENV = <(X, var), (Y, var)>; out TREE)
              GET_TOKEN (TOKEN = X)
              STATEMENT (in ENV = <(X, var), (Y, var)>; out TREE)
                GET_TOKEN (TOKEN = :=)
                GET_TOKEN (TOKEN = 0)
                EXPRESSION (in ENV = <(X, var), (Y, var)>; out TREE)
                  TERM (in ENV = <(X, var), (Y, var)>; out TREE)
                    FACTOR (in ENV = <(X, var), (Y, var)>; out TREE)
                      GET_TOKEN (TOKEN = ;)
                      return TREE = (0)
                    return TREE = (0)
                  return TREE = (0)
                return TREE = (:= X 0)
              GET_TOKEN (TOKEN = Y)
              STATEMENT (in ENV = <(X, var), (Y, var)>; out TREE)
                GET_TOKEN (TOKEN = :=)
                GET_TOKEN (TOKEN = X)
                EXPRESSION (in ENV = <(X, var), (Y, var)>; out TREE)
                  TERM (in ENV = <(X, var), (Y, var)>; out TREE)
                    FACTOR (in ENV = <(X, var), (Y, var)>; out TREE)
                      GET_TOKEN (TOKEN = +)
                      return TREE = (X)
                    return TREE = (X)
                  GET_TOKEN (TOKEN = 1)
                  TERM (in ENV = <(X, var), (Y, var)>; out TREE)
                    FACTOR (in ENV = <(X, var), (Y, var)>; out TREE)
                      GET_TOKEN (TOKEN = end)
                      return TREE = (1)
                    return TREE = (1)
                  return TREE = (+ X 1)
                return TREE = (:= (+ X 1))
              GET_TOKEN (TOKEN = .)
              return TREE = (; (:= X 0) (:= (+ X 1)))
            return ENV = <(X, var), (Y, var)>, TREE = (; (:= X 0) (:= (+ X 1)))
          return TREE = (; (:= X 0) (:= (+ X 1)))

        Note: Indentation level identifies function components.