  Let $\mathcal{M}^{\mathrm{st}}$ ($\mathcal{M}^{\mathrm{pst}}$) be a moduli
space of stable (polystable) bundles with fixed determinant on a complex
surface with $b_1=1$, $p_g=0$, and let $Z\subset \mathcal{M}^{\mathrm{st}}$ be
a pure $k$-dimensional analytic set. We prove a general formula for the
homological boundary $\delta[Z]^{BM}\in H_{2k-1}^{BM}(\partial\hat{\mathcal
M}^{\mathrm{pst}},\mathbb{Z})$ of the Borel-Moore fundamental class of $Z$ in
the boundary of the blow up moduli space $\hat {\mathcal M}^{\mathrm{pst}}$.
The proof is based on the holomorphic model theorem (proved in a previous
article), which identifies a neighborhood of a boundary component of $\hat
{\mathcal M}^{\mathrm{pst}}$ with a neighborhood of the boundary of a "blow up
flip passage".
  We then focus on a particular instanton moduli space which intervenes in our
program for proving the existence of curves on class VII surfaces. Using our
result, combined with general properties of the Donaldson cohomology classes,
we prove incidence relations between the Zariski closures (in the considered
moduli space) of certain families of extensions. These incidence relations are
crucial for understanding the geometry of the moduli space, and cannot be
obtained using classical complex geometric deformation theory.
