code: public function addPlaceholder($placeholder, $description, $sampleValue, $emptyValue)
      {
          if (isset($this->placeholders[$placeholder])) {
              throw new DefinitionDuplicateException(
                  sprintf('Field with same placeholder "%s" already exists in structure', $placeholder)
              );
          }
  
          $this->placeholders[$placeholder] = array(
              'placeholder' => $placeholder,
              'description' => $description,
              'sample' => $sampleValue,
              'empty' => $emptyValue,
          );
      }
docs: Add placeholder definition to structure
  
  @param string $placeholder
  @param string $description
  @param string $sampleValue
  @param string $emptyValue
  
  @throws DefinitionDuplicateException if new definition conflicts with existing one