estor container of original end node,
  // let first partially contained child be null."
  var firstPartiallyContainedChild;
  if (isAncestorContainer(originalStartNode, originalEndNode)) {
    firstPartiallyContainedChild = null;
  // "Otherwise, let first partially contained child be the first child of
  // common ancestor that is partially contained in the context object."
  } else {
    for (var i = 0; i < commonAncestor.childNodes.length; i++) {
      if (isPartiallyContained(commonAncestor.childNodes[