XmlNode = type {
        tag: string; 
        attrs: [string]; 
        content: string}.
        
Tree = type(Leaf) Tree [Leaf; [Tree(Leaf)]].

XmlTree = Tree(XmlNode).

query = function(tree: XmlTree): [XmlTree]
{
    children :  context(children) = 
      case default
        []
      
      case (tree[0]: NeedChildren(childrenFilters), ApplyTransform(transFilters))
      {
        loop fold (tree[1]->child: Tree(Leaf) | childrenFilters; []->accum).
            {accum  + traverse(child): cases(transFilters) };
      }

    node = 
      case (node.tag == "style:style"; class = node[attrs, "style:name"]; exists(class, ParaKnownClasses))
      {
        [class, node[attrs, "style:display-name"]];
      }
      
      case default
        [] ;
      
      Tree[node children] | node + children
}

  /*
traverse = function(trees: [XmlTree]) : [XmlTree]
  loop fold(trees->tree: Tree; []->acc) 
    acc + traverse(tree)
   */