Thank you for this suggestion! There is a problem implementing this because here the `ReturnsSelf()` extension method will not have a reference to self (`simple`). It will only get a reference to whatever `Do()` returns (in this case it will default to a new substitute).
We do have a different feature designed to help with these sort of builder patterns which you might be interested in instead. There is an extension called `ReturnsForAll<T>(T value)`, which will make all methods on a substitute that return `T` return the specific `value`. You will first need to bring this into scope with `using NSubstitute.Extensions;`. Here is an example: