type
    Node = struct {
       int val;
       *Node left, right;
    },
    Tree = *Node;

proc tree(int v; Tree l, r) Tree:
    Tree t;
    t := new(Node);
    t*.val := v;
    t*.left := l;
    t*.right := r;
    t
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proc preorder(Tree t) void:
    if t /= nil then
        write(t*.val, ' ');
        preorder(t*.left);
        preorder(t*.right)
    fi
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proc inorder(Tree t) void:
    if t /= nil then
        inorder(t*.left);
        write(t*.val, ' ');
        inorder(t*.right)
    fi
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proc postorder(Tree t) void:
    if t /= nil then
        postorder(t*.left);
        postorder(t*.right);
        write(t*.val, ' ')
    fi
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proc levelorder(Tree t) void:
    [256]Tree q;
    word s, e;
    s := 0;
    q[s] := t;
    e := 1;
    while s /= e do
       if q[s] /= nil then
          q[e] := q[s]*.left;
          q[e+1] := q[s]*.right;
          e := e+2;
          write(q[s]*.val, ' ')
       fi;
       s := s+1
    od
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proc main() void:
    Tree t;
    t := tree(1,
              tree(2,
                   tree(4,
                        tree(7,nil,nil),
                        nil),
                   tree(5,nil,nil)),
              tree(3,
                   tree(6,
                        tree(8,nil,nil),
                        tree(9,nil,nil)),
                   nil));

    write("preorder:    "); preorder(t); writeln();
    write("inorder:     "); inorder(t); writeln();
    write("postorder:   "); postorder(t); writeln();
    write("level-order: "); levelorder(t); writeln();
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