text: from fastapi import FastAPI
  
  app = FastAPI()
  
  
  @app.get("/app")
  def read_main():
      return {"message": "Hello World from main app"}
  
  
  subapi = FastAPI()
  
  
  @subapi.get("/sub")
  def read_sub():
      return {"message": "Hello World from sub API"}
  
  
  app.mount("/subapi", subapi)
  