code: public void swapTwoChildren(GroovySourceAST t) {
          // this swaps the two child nodes, see javadoc above for explanation of implementation
          GroovySourceAST a = (GroovySourceAST) t.getFirstChild();
          GroovySourceAST b = (GroovySourceAST) a.getNextSibling();
  
          t.setFirstChild(b);
          a.setNextSibling(null);
          b.setNextSibling(a);
      }
docs: To swap two children of node t...
  
  <pre>
  (t)
  |
  |
  (a) -- (b)
  
  t.down = firstNode
  a.right = b
  b.right = null
  </pre>
  becomes
  <pre>
  (t)
  |
  |
  (b) -- (a)
  
  t.down = b
  a.right = null
  b.right = a
  </pre>
  
  todo - build API of basic tree mutations like this method.