public class Demo2 {
    public static void main(String[] args){

        Tree sampleTree = new Tree("dog",
                                   new Tree("bat",
                                            null,
                                            new Tree("camel", null, null)),
                                   new Tree("mouse", null, null));
        
        // The outermost procedure has a static link pointing at a frame
        //  containing any global variables.
        GlobalFrame globals = new GlobalFrame();
        
        String result = prettyprint(new PrettyprintFrame(globals),
                                    sampleTree);

        System.out.println(result);
    }

    static String prettyprint(PrettyprintFrame fp,
                              Tree tree){
        fp.output = "";
        show(new ShowFrame(fp),
             0, tree);
        return fp.output;
    }

    static void write(WriteFrame fp,
                      String s){
        fp.staticLink.output = fp.staticLink.output + s;
    }

    static void show(ShowFrame fp,
                     int nArg, Tree t){

        fp.n = nArg;  // store in frame since it escapes

        if(t == null){
            indent(new IndentFrame(fp),
                   ".");
        } else {
            indent(new IndentFrame(fp),
                   t.key);
            show(new ShowFrame(fp.staticLink),
                 fp.n + 1, t.left);
            show(new ShowFrame(fp.staticLink),
                 fp.n + 1, t.right);
        }
    }

    static void indent(IndentFrame fp,
                       String s){
        for(int i = 1; i <= fp.staticLink.n; i++){
            write(new WriteFrame(fp.staticLink.staticLink),
                  " ");
        }
        fp.staticLink.staticLink.output = fp.staticLink.staticLink.output + s;
        write(new WriteFrame(fp.staticLink.staticLink),
              "\n");
    }
}

class Tree {
    String key;
    Tree left, right;
    Tree(String key, Tree left, Tree right){
        this.key = key;
        this.left = left;
        this.right = right;
    }
}

class GlobalFrame {
    // any globals would go in here; Program 6.3 has none
}

class PrettyprintFrame {
    GlobalFrame staticLink;
    
    String output;  // this is the one escaping variable
    
    PrettyprintFrame(GlobalFrame staticLink){
        this.staticLink = staticLink;
    }
}

class WriteFrame {
    PrettyprintFrame staticLink;
    
    // no escaping variables
    
    WriteFrame(PrettyprintFrame staticLink){
        this.staticLink = staticLink;
    }
}

class ShowFrame {
    PrettyprintFrame staticLink;
    
    int n; // escaping variable
    
    ShowFrame(PrettyprintFrame staticLink){
        this.staticLink = staticLink;
    }
}

class IndentFrame {
    ShowFrame staticLink;

    // no escaping variables

    IndentFrame(ShowFrame staticLink){
        this.staticLink = staticLink;
    }
}