How to generate AST using Xtext and Xtend?

Question

So I have the default example of a xtext grammar.

grammar org.xtext.example.mydsl.MyDsl with org.eclipse.xtext.common.Terminals

generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"

Model:
    greetings+=Greeting*;

Greeting:
    'Hello' name=ID '!';

I'd like to view the abstract syntax tree - in particular I'd like the corresponding xtend file, which looks like

package org.xtext.example.mydsl.generator

import org.eclipse.emf.ecore.resource.Resource
import org.eclipse.xtext.generator.IGenerator
import org.eclipse.xtext.generator.IFileSystemAccess

class MyDslGenerator implements IGenerator {

    override void doGenerate(Resource resource, IFileSystemAccess fsa) {
        //TODO implement me
    }
}

To printout a textual version of the AST whenever doGenerate is called. I've been hacking at this for about two days and I'm clearly missing something fundamental - someone take pity on me? What would go in the doGenerate Function?