code: function deepCollection(collectionId, obj) {
    const pathBits = idKeys(collectionId);
    pathBits.pop();
    pathBits.push('collection');
    pathBits.shift();
    return deepObj(pathBits, obj, false);
  }
docs: Given a nested pattern `obj` and a `patternId`, find its collection
  in the object `obj`.
  @see deepPattern