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,
        );
    }