code: function nodeWalkPrior(node, predicate) {
    // Search our earlier siblings and their descendents.
    var parent = node.parentNode;
    if (parent && parent.childNodes) {
      var idx = parent.childNodes.indexOf(node);
      var siblings = parent.childNodes.slice(0, idx);
      for (var i = siblings.length - 1; i >= 0; i--) {
        var sibling = siblings[i];
        if (predicate(sibling)) {
          return sibling;
        }
        var found = nodeWalk(sibling, predicate);
        if (found) {
          return found;
        }
      }
      if (predicate(parent)) {
        return parent;
      }
      return nodeWalkPrior(parent, predicate);
    }
    return undefined;
  }
docs: Equivalent to `nodeWalk`, but only returns nodes that are either
  ancestors or earlier cousins/siblings in the document.
  
  Nodes are searched in reverse document order, starting from the sibling
  prior to `node`.