langchain/chains/combine_documents/map_reduce.py/MapReduceDocumentsChain/get_default_document_variable_name
class MapReduceDocumentsChain:    def get_default_document_variable_name(cls, values: Dict) -> Dict:
        """Get default document variable name, if not provided."""
        if "document_variable_name" not in values:
            llm_chain_variables = values["llm_chain"].prompt.input_variables
            if len(llm_chain_variables) == 1:
                values["document_variable_name"] = llm_chain_variables[0]
            else:
                raise ValueError(
                    "document_variable_name must be provided if there are "
                    "multiple llm_chain input_variables"
                )
        else:
            llm_chain_variables = values["llm_chain"].prompt.input_variables
            if values["document_variable_name"] not in llm_chain_variables:
                raise ValueError(
                    f"document_variable_name {values['document_variable_name']} was "
                    f"not found in llm_chain input_variables: {llm_chain_variables}"
                )
        return values