ParseTree aufbauen will nicht so recht

[jkallup]

Hallo,

wie kann ich einen Baum aufstellen?
Irgendwo ist da noch ein Fehler im Code:

    type
      TParserTree = class;
      TTreeType = (OperatorNode, NumberNode, VariableNode);
      TOperator = record
        left: TParserTree;
        right: TParserTree;
        operator: Char;
      end;
      TParserTree = class
        ntype: TTreeType;
        body: TOperator;
        value: Variant;
      end;
     
    var
      tree, pleft, pright: TParserTree;
     
    function make_operator(left: TParserTree; op: Char; right: TParserTree): TParserTree;
    begin
      result := TParserTree.Create;
      result.ntype := operatorNode;
      result.body.left := left;
      result.body.left.body.operator := op;
      result.body.right := right;
    end;
     
    function make_number(n: Integer): TParserTree;
    begin
      result := TParserTree.Create;
      result.ntype := numberNode;
      result.value := n;
    end;
     
    function make_variable (v: String): TParserTree;
    begin
      result := TParserTree.Create;
      result.ntype := variableNode;
      result.value := v;
    end;
     
    function printTree(t: TParserTree; level: Integer): String;
    var
      step, i: Integer;
      s: String;
    begin
      step := 4;
     
      for i := 1 to level+step do
      s := s + ' ';
     
      if Assigned(t) then
      begin
        case t.ntype of
          operatorNode:
          begin
            result := result + printTree(t.body.right, level+step);
            result := result + Format('%s %s'+#10, [s,char(t.body.operator)]);
            result := result + printTree(t.body.left, level+step);
          end;
     
          numberNode: begin
            result := result + Format('%s %d', [s, Integer(t.value)]);
          end;
     
          variableNode: begin
          showmessage('variableNode;  ' + t.value);
            result := result + printTree(t.body.right, level+step);
            result := result + Format('%s %s', [s, String(t.value)]);
            result := result + printTree(t.body.left , level+step);
          end;
        end;
      end;
    end;
     
    begin
        pleft  := TParserTree.Create; pleft .value := 2; pleft .ntype := numberNode;
        pright := TParserTree.Create; pright.value := 3; pright.ntype := numberNode;
        tree   := make_operator(pleft,'+',pright);
     
        pl2  := TParserTree.Create; pl2.value := 4; pl2.ntype := numberNode;
        tree := make_operator(tree,'*',pl2);
     
        ShowMessage('1. Tree:'+#10+printTree(tree,4));
    end;