langchain/docstore/in_memory.py/InMemoryDocstore/add
class InMemoryDocstore:    def add(self, texts: Dict[str, Document]) -> None:
        """Add texts to in memory dictionary."""
        overlapping = set(texts).intersection(self._dict)
        if overlapping:
            raise ValueError(f"Tried to add ids that already exist: {overlapping}")
        self._dict = dict(self._dict, **texts)