Example: ``` const mount = require('koa-mount'); const Koa = require('koa'); const a = new Koa(); a.context.test = "hallo"; a.use(async function (ctx, next){ ctx.body = "test:" + ctx.test; return next(); }); // app const app = new Koa(); app.use(mount('/hello', a)); app.listen(3000) a.listen(3001); ``` http://localhost:3000/hello results in `test:hallo` http://localhost:3001/hello results in `test:undefined`