code: boolean deleteIndexEntry(S userStorable) throws PersistException {
          try {
              return makeIndexEntry(userStorable).tryDelete();
          } catch (PersistException e) {
              Throwable cause = e.getCause();
              if (cause instanceof IllegalArgumentException) {
                  // Can be caused by a corrupt master record, which is
                  // attempting do assign an illegal value to the index. There's
                  // no way to find the old index entry to delete.
                  return false;
              }
              throw e;
          }
      }
docs: Assumes caller is in a transaction