“…In practice, the SOLVE module may become computationally expensive (in contrast to all other modules) when employing a direct solver; see, e.g., [18,22,29] for a discussion of implementational aspects. Thus, usually, an iterative solver is employed to compute an approximation 𝑢 𝐿 of 𝑢 ⋆ 𝐿 on each level, and the exact Galerkin solution 𝑢 ⋆ 𝐿 is not available.…”