def message_multiline(key)
      begin
        skip_whitespace
        if @scanner.check(/"/)
          add_result(key, message_line)
          message_multiline(key)
        end
      rescue PoSyntaxError => pe
        raise PoSyntaxError, "Syntax error in message_multiline with key '#{key}'\n" + pe.message, pe.backtrace
      end
    end
def message_multiline(key)
      begin
        skip_whitespace
        if @scanner.check(/"/)
          add_result(key, message_line)
          message_multiline(key)
        end
      rescue PoSyntaxError => pe
        raise PoSyntaxError, "Syntax error in message_multiline with key '#{key}'\n" + pe.message, pe.backtrace
      end
    end
parses a multiline message

 multiline messages are indicated by an empty content as first line and the next line
 starting with the double quote character