// ^(nil 5) becomes new root of ^(nil 101 102 103)
CommonTree newRoot = new CommonTree((Token)null);
newRoot.addChild(new CommonTree(new CommonToken(5)));
CommonTree oldRoot = new CommonTree((Token)null);
oldRoot.addChild(new CommonTree(new CommonToken(101)));
oldRoot.addChild(new CommonTree(new CommonToken(102)));
oldRoot.addChild(new CommonTree(new CommonToken(103)));
TreeAdaptor adaptor = new CommonTreeAdaptor();
adaptor.becomeRoot(newRoot, oldRoot);