docs: public void mergeNodes( DdlTokenStream tokens,
                              AstNode firstNode,
                              AstNode secondNode ) {
          assert tokens != null;
          assert firstNode != null;
          assert secondNode != null;
  
          int firstStartIndex = (Integer)firstNode.getProperty(DDL_START_CHAR_INDEX);
          int secondStartIndex = (Integer)secondNode.getProperty(DDL_START_CHAR_INDEX);
          int deltaLength = ((String)secondNode.getProperty(DDL_EXPRESSION)).length();
          Position startPosition = new Position(firstStartIndex, 1, 0);
          Position endPosition = new Position((secondStartIndex + deltaLength), 1, 0);
          String source = tokens.getContentBetween(startPosition, endPosition);
          firstNode.setProperty(DDL_EXPRESSION, source);
          firstNode.setProperty(DDL_LENGTH, source.length());
      }