code: private function setBaseEval($fieldDefinition, $overrides)
      {
          if ($this->isAllowedValue('isunique')) {
              $fieldDefinition['eval']['unique'] = (bool) $this->getOverrideValue('isunique', $overrides);
          }
  
          foreach (array(
              'tl_class',
              'mandatory',
              'alwaysSave',
              'chosen',
              'allowHtml',
              'preserveTags',
              'decodeEntities',
              'rte',
              'rows',
              'cols',
              'spaceToUnderscore',
              'includeBlankOption',
              'submitOnChange',
              'readonly'
          ) as $name) {
              if (empty($fieldDefinition['eval'][$name])
                  && ($value = $this->getOverrideValue($name, $overrides))
              ) {
                  $fieldDefinition['eval'][$name] = $value;
              }
          }
  
          // If we have unique, enforce mandatory.
          if (!empty($fieldDefinition['eval']['unique'])) {
              $fieldDefinition['eval']['mandatory'] = true;
          }
  
          return $fieldDefinition;
      }
docs: Extract an override value.
  
  @param array $fieldDefinition The field definition.
  
  @param array $overrides       The overrides containing the values to be overridden.
  
  @return array