last partially contained child be the last child of
  // common ancestor that is partially contained in the context object."
  } else {
    for (var i = commonAncestor.childNodes.length - 1; i >= 0; i--) {
      if (isPartiallyContained(commonAncestor.childNodes[i], range)) {
        lastPartiallyContainedChild = commonAncestor.childNodes[i];
        break;
      }
    }
    if (!lastPartiallyContainedChild) {
      throw "Spec bug: no last partially contained child!";
    }
  }

  // "Let contained childre