PL/0 ATTRIBUTED PARSE TREE EXAMPLE
                       ----------------------------------

Program:

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


                                <program>
                              44) env:  e0
                              45) tree: t0
                                /       \
                               /         \
                              /           \
                        <block>            .
                      8) env:  e0
                     43) tree: t0
                     /|         |\
                    / |         | \
                   /  |         |  \
                  /   |         |   \
                 /    |         |    \
                /     |         |     \
               /      |         |      \
              /       |         |       \
             /        |         |        \
            /         |         |         \
           /          |         |          \
          /           |         |           \
         /            |         |            \
        /             |         |             \
       /              |         |              \
      /               |         |               \
<const-decl>      <var-decl>    <proc-decl>  <statement>
1) env: <>        6) env: e0    7) env: <>   9) env:  e0
     |            /    |    \        |      42) tree: t0
     |           /     |     \       |            |
     e          /      |      \      e            |
               /       |       \                  |
              /        |        \                 |
             /         |         \                |
            /          |          \               |
           var    <ident-list>     ;              |
                   5) env: e0                     |   e0 = <(X, var), (Y, var)>
                  /    |     \                    |
                 /     |      \                   |
                /      |       \                  |
               /       |        \                 |
              /        |         \                |
             /         |          \               |
       <ident-list>    ,        <ident>           |
    3) env: <(X, var)>        4) lexeme: Y        |
            |                      |              |
            |                      |              |
         <ident>                   Y              |
       2) lexeme: X                               |
            |                                     |
            |                                     |
            X                                     |
                                                  |
                                                  |
                                                  |
                                                  |
        t0 = ;                                   /|\
            / \                                 / | \
           /   \                               /  |  \
          /     \                             /   |   \
         /       \                           /    |    \
        :=       :=                         /     |     \
        /\       /\                        /      |      \
       /  \     /  \                      /       |       \
      X    0   Y    +                    /        |        \
                   / \                  /         |         \
                  /   \              begin <statement-list> end
                 X     1                     10) env:  e0
                                             41) tree: t0
                                           /      |       \
                                          /       |        \
                                         /        |         \
                                 <statement-list> ;     <statement>
                                   11) env:  e0         12) env:  e0
        t3 = :=                    23) tree: t1         40) tree: t3
             /\                          |                   |
            /  \                         |                   |
           Y    +                        |                   |
               / \                       |                   |
              /   \                      |                   |
             X     1                     |                   |
                                    <statement>              |
                                    13) env:  e0             |
        t1 = :=                     22) tree: t1             |
             /\                     /    |    \              |
            /  \                   /     |     \             |
           X    0                 /      |      \            |
                                 /       |       \           |
                                /        |        \          |
                               /         |         \         |
                           <ident>      :=    <expression>   |
                        14) lexeme: X         15) env:  e0   |
                              |               21) tree: 0    |
                              |                     |        |
                              |                     |        |
                              X                  <term>      |
                                              16) env:  e0   |
                                              20) tree: 0    |
                                                    |        |
                                                    |        |
                                                <factor>     |
                                              17) env:  e0   |
                                              19) tree: 0    |
                                                    |        |
                                                    |        |
                                                <number>     |
                                              18) lexeme: 0  |
                                                    |        |
                                                    |        |
                                                    0        |
                                                             |
                                                             |
                                                             |
                                                             /
                                                            /
                                                           /
                                                          /
                                                         /
                                                        /
                                                       /
                                                      /
                                                     /
                                                    /
                                                   /
                                                  /
                                                 /
                                                /
                                               /
                                              /
                                             /
                                            /
                                           /
                                          /
                                         /
                                        /
                                        |
                                        |
                                        |
                                       /|\
                                      / | \
                                     /  |  \
                                    /   |   \
                                   /    |    \
                                  /     |     \
                                 /      |      \
                                /       |       \
                               /        |        \
                              /         |         \
                             /          |          \
                            /           |           \
                           /            |            \
                          /             |             \
                         /              |              \
                     <ident>           :=         <expression>
                  24) lexeme: Y                   25) env:  e0
                        |                         39) tree: t2       t2 = +
                        |                         /     |    \           / \
                        |                        /      |     \         /   \
                        Y                       /       |      \       X     1
                                               /        |       \
                                              /         |        \
                                             /          |         \
                                            /           |          \
                                           /            |           \
                                          /             |            \
                                         /              |             \
                                  <expression>  <adding operator>   <term>
                                  26) env:  e0    34) lexeme: +   27) env:  e0
                                  33) tree: X           |         38) tree: 1
                                        |               |              |
                                        |               |              |
                                     <term>             +           <factor>
                                  28) env:  e0                    35) env:  e0
                                  32) tree: X                     37) tree: 1
                                        |                              |
                                        |                              |
                                    <factor>                        <number>
                                  29) env:  e0                    36) lexeme: 1
                                  31) tree: X                          |
                                        |                              |
                                        |                              |
                                     <ident>                           1
                                  30) lexeme: X
                                        |
                                        |
                                        X