The interface of the `InverseOperator::apply` has two overloads, one where the target reduction is dictated by the internal member object `_reduction` and another one where the target reduction is dictated by a third extra argument of `InverseOperator::apply`. The idea is that the extra argument supersedes the internal one. This works for most of the solvers, except in [`Restarted[Flexible]GMRes`](https://gitlab.dune-project.org/core/dune-istl/-/blob/4f1342be8eb2d30a2d84d12627bea3fa94d47b71/dune/istl/solvers.hh#L924) where this argument is completely ignored and the reduction is made with the internal member object. If the user request a specific reduction via this interface, it either wastes resources on a stricter internal reduction, or does not solve the system with enough accuracy.