/**
  * Merge fields information
  */
 private function _mergeFields()
 {
     foreach ($this->_segmentInfos as $segName => $segmentInfo) {
         foreach ($segmentInfo->getFieldInfos() as $fieldInfo) {
             $this->_fieldsMap[$segName][$fieldInfo->number] = $this->_writer->addFieldInfo($fieldInfo);
         }
     }
 }