title: Firestore security rules: check if array contains strings different from user's ID
body: <p>I know how to check if an array contains a given string (as explained for example <a href="https://stackoverflow.com/questions/46835481/firestore-security-rules-searching-for-a-users-id-in-array-in-a-document">here</a>). My requirement however is different: I have a document with an array <em>updatedByHistoryArray</em> written at server side that contains the history of the ids of all users who updated such a document, for example <em>[id1, id2, ..., idn]</em>.</p>
  
  <p>I would like to allow a delete operation for this document only if the latter has been updated exclusively by the user who wants to delete it.
  So, for example, if a user with id24 wants to delete a document, the updatedByHistoryArray of this document has to be [id24, id24, ..., id24].</p>
  
  <p>Is it possible to implement this requirement in the security rules of Firestore?</p>