code: @NonNull
      public static Expression rank(@NonNull String indexName) {
          if (indexName == null) {
              throw new IllegalArgumentException("indexName cannot be null.");
          }
          return new Expression.FunctionExpression("RANK()", Arrays.asList(Expression.string(indexName)));
      }
docs: Creates a full-text rank function with the given full-text index name.
  The rank function indicates how well the current query result matches
  the full-text query when performing the match comparison.
  
  @param indexName The index name.
  @return The full-text rank function.