  Let $G=(V,E)$ be an undirected multi-graph on $n=|V|$ vertices and
$S\subseteq V$ be a Steiner set. Steiner cut is a fundamental concept;
moreover, global cut $(|S|=n)$, as well as (s,t)-cut $(|S|=2)$, is just a
special case of Steiner cut. We study Steiner cuts of capacity minimum+1, and
as an important application, we provide a dual edge Sensitivity Oracle for
Steiner mincut.
  A compact data structure for cuts of capacity minimum+1 has been designed for
both global cuts [STOC 1995] and (s,t)-cuts [TALG 2023]. Moreover, both data
structures are also used crucially to design a dual edge Sensitivity Oracle for
their respective mincuts. Unfortunately, except for these two extreme scenarios
of Steiner cuts, no generalization of these results is known. Therefore, to
address this gap, we present the following first results on Steiner cuts.
  1. Data Structure: There is an $O(n(n-|S|+1))$ space data structure that can
determine in $O(1)$ time whether a given pair of vertices is separated by a
Steiner cut of capacity at least minimum+1. It can report such a cut, if it
exists, in $O(n)$ time.
  2. Sensitivity Oracle: (a) There is an $O(n(n-|S|+1))$ space data structure
that, after the failure/insertion of any pair of edges, can report the capacity
of Steiner mincut in $O(1)$ time and a Steiner mincut in $O(n)$ time. (b) If we
are interested in reporting only the capacity, there is a more compact data
structure that occupies $O((n-|S|)^2+n)$ space and reports the capacity in
$O(1)$ time after the failure/insertion of any pair of edges.
  3. Lower Bound: For undirected multi-graphs, for every Steiner set $S$, any
data structure that, after the failure or insertion of any pair of edges, can
report the capacity of Steiner mincut must occupy $\Omega((n-|S|)^2)$ bits of
space, irrespective of the query time.
