"""Combine two prompt templates.
        Args:
            other: Another prompt template.
        Returns:
            Combined prompt template.
        """
        # Allow for easy combining
        if isinstance(other, ChatPromptTemplate):
            return ChatPromptTemplate(messages=self.messages + other.messages)
        elif isinstance(
            other, (BaseMessagePromptTemplate, BaseMessage, BaseChatPromptTemplate)
        ):
            return ChatPromptTemplate(messages=self.messages + [other])
        elif isinstance(other, str):
            prompt = HumanMessagePromptTemplate.from_template(other)
            return ChatPromptTemplate(messages=self.messages + [prompt])
        else:
            raise NotImplementedError(f"Unsupported operand type for +: {type(other)}")
    @root_validator(pre=True)
    def validate_input_variables(cls, values: dict) -> dict:
        """Validate input variables.
        If input_variables is not set, it will be set to the union of
        all input variables in the messages.
        Args:
            values: values to validate.
        Returns:
            Validated values.
        """
        messages = values["messages"]
        input_vars = set()
        for message in messages:
            if isinstance(message, (BaseMessagePromptTemplate, BaseChatPromptTemplate)):
                input_vars.update(message.input_variables)
        if "partial_variables" in values:
            input_vars = input_vars - set(values["partial_variables"])
        if "input_variables" in values:
            if input_vars != set(values["input_variables"]):
                raise ValueError(
                    "Got mismatched input_variables. "
                    f"Expected: {input_vars}. "
                    f"Got: {values['input_variables']}"
                )
        else:
            values["input_variables"] = list(input_vars)
        return values