  For a partition $\beta$, denote by $N_\beta$ the nilpotent linear operator of
Jordan type $\beta$. Given partitions $\beta$, $\gamma$, we investigate the
representation space ${}_2{\mathbb V}_{\gamma}^\beta$ of all short exact
sequences $$ \mathcal E: 0\to N_\alpha \to N_\beta \to N_\gamma \to 0$$ where
$\alpha$ is any partition with each part at most 2. Due to the condition on
$\alpha$, the isomorphism type of a sequence $\mathcal E$ is given by an arc
diagram $\Delta$; denote by ${\mathbb V}_\Delta$ the subset of ${}_2{\mathbb
V}_{\gamma}^\beta$ of all sequences isomorphic to $\mathcal E$.
  Thus, the space ${}_2{\mathbb V}_{\gamma}^\beta$ carries a stratification
given by the subsets of type ${\mathbb V}_\Delta$. We compute the dimension of
each stratum and show that the boundary of a stratum ${\mathbb V}_\Delta$
consists exactly of those ${\mathbb V}_{\Delta'}$ where $\Delta'$ is obtained
from $\Delta$ by a non-empty sequence of arc moves of five possible types {\bf
(A) -- (E)}.
  The case where all three partitions are fixed has been studied in [3] and
[4]. There, arc moves of types {\bf (A) -- (D)} suffice to describe the
boundary of a ${\mathbb V}_\Delta$ in ${\mathbb V}_{\alpha,\gamma}^\beta$. Our
fifth move {\bf (E)}, "explosion", is needed to break up an arc into two poles
to allow for changes in the partition $\alpha$.
