How to find elements that are not deeper than a selector?

I am building a jQuery plugin to manage form collections. The plugin aims to add add, remove, move up and move down buttons to alter that collection.

A collection's root node always contains a selector, such as .collection.

I implemented min and max options, so add and remove buttons disappear accordingly. My problem comes up when I try to manage a collection of form collections: how to select only the add buttons that refers to the right collection?

To simplify the problem, look at the following HTML code:

<div class="collection">
  <div>something</div>
  <div>something</div>
  <div>
    <div class="add">+</div>
  </div>
  <div>something</div>
  <div class="collection">
    <div>something</div>
    <div>something</div>
    <div>
      <div class="add">+</div>
    </div>
    <div>something</div>
  </div>
</div>

Keep in mind that the button can be arbitrary deep: collection is built by an user and I don't know where can be the button in the dom. BTW, it is deeper than the .collection, that's all I know.

How to select all add buttons until the second .collection, but not further?