Hmmm - `routerType` and `routerTypeChain` are the same except `routerType` allows a `poller`.

Also, the documentation is missing for `expression` in `routerTypeChain`. Seems to me we can factor out these common elements into a group... https://github.com/garyrussell/spring-integration/commit/cb0cd4c1d65ccfe608181b95f751356cf877d99f

I am not sure why we didn't do that when fixing the poller within the chain.