langchain/prompts/example_selector/ngram_overlap.py/NGramOverlapExampleSelector/check_dependencies
class NGramOverlapExampleSelector:    def check_dependencies(cls, values: Dict) -> Dict:
        """Check that valid dependencies exist."""
        try:
            from nltk.translate.bleu_score import (  # noqa: disable=F401
                SmoothingFunction,
                sentence_bleu,
            )
        except ImportError as e:
            raise ValueError(
                "Not all the correct dependencies for this ExampleSelect exist"
            ) from e

        return values