code: public TreeNode queryForSingleton(Query query) throws IllegalStateException {
      return query(queryBuilder().chain(query).ensureUnique().build()).iterator().next();
    }
docs: Return the unique node selected by running this query against this
  {@code ContextManager}'s root context.
  <p>
  If this query does not return a single unique result then an
  {@code IllegalStateException} will be thrown.  More details on the query
  execution context can be found in {@link #query(Query)}.
  
  @param query the query to execute
  @return the node selected by the query
  @throws IllegalStateException if the query does not select a unique node
  @see #query(Query)
  @see QueryBuilder#ensureUnique()