code: public void append(String asciiString) {
      if (asciiString == null) {
        throw new IllegalArgumentException("AsciiString must not be null");
      }
      ANTLRInputStream antlrInputStream = new ANTLRInputStream(asciiString);
      GDLLexer lexer = new GDLLexer(antlrInputStream);
      GDLParser parser = new GDLParser(new CommonTokenStream(lexer));
      // update the loader state while walking the parse tree
      new ParseTreeWalker().walk(loader, parser.database());
    }
docs: Append the given GDL string to the current database.
  
  @param asciiString GDL string (must not be {@code null}).