langchain/memory/combined.py/CombinedMemory/memory_variables
class CombinedMemory:    def memory_variables(self) -> List[str]:
        """All the memory variables that this instance provides."""
        """Collected from the all the linked memories."""

        memory_variables = []

        for memory in self.memories:
            memory_variables.extend(memory.memory_variables)

        return memory_variables