$source = visitNode($source, function($node){
    if($node instanceof Namespace_) return NULL;
    return NodeTraverser::DONT_TRAVERSE_CHILDREN;
}, function($node){
    if($node instanceof Name) return NULL; // this is the name of Namespace_
    if($node instanceof Namespace_) return NULL;
    if($node instanceof Declare_) return NULL;
    if($node instanceof Use_) return NULL;
    if($node instanceof Class_) return NULL;
    if($node instanceof Interface_) return NULL;
    if($node instanceof Function_) return NULL;
    return NodeTraverser::REMOVE_NODE;
});