  Let $k$ be an algebraically closed field and $\alpha$, $\beta$, $\gamma$ be
partitions. An algebraic group acts on the constructible set of short exact
sequences of nilpotent $k$-linear operators of Jordan types $\alpha$, $\beta$,
and $\gamma$, respectively; we are interested in the stratification given by
the orbits in the case where all parts of $\alpha$ are at most 2. Geometric
properties of the degeneration relation are controlled by the combinatorics of
arc diagrams. The extended bubble sort algorithm is used to construct chains of
orbits such that subsequent strata have dimension difference equal to one.
