Update app.py

app.py

@@ -17,6 +17,7 @@ from langchain_community.document_loaders.directory import DirectoryLoader
 from langchain_core.output_parsers import StrOutputParser
 from langchain_core.runnables import RunnablePassthrough
 from langchain_experimental.chat_models import Llama2Chat
+from langchain_experimental.chat_models.system_messages import SystemMessage
 
 lang_api_key = os.getenv("lang_api_key")