  Recent work uncovered features in the phase space of the Milky Way's stellar
halo which may be attributed to the last major merger. When stellar material
from a satellite is accreted onto its host, it phase mixes and appears finely
substructured in phase space. For a high-eccentricity merger, this substructure
most clearly manifests as numerous wrapping chevrons in $(v_r, r)$ space,
corresponding to stripes in $(E, \theta_r)$ space. We introduce the idea of
using this substructure as an alternative subhalo detector to cold stellar
streams. We simulate an N-body merger akin to the GSE and assess the impact of
subhaloes on these chevrons. We examine how their deformation depends on the
mass, pericentre, and number of subhaloes. To quantify the impact of
perturbers, we utilise the appearance of chevrons in $(E, \theta_r)$ space to
introduce a new quantity -- the ironing parameter. We show that: (1) a single
flyby of a massive ($\sim 10^{10}$ M$_{\odot}$) subhalo with pericentre
comparable to, or within, the shell's apocentre smooths out the substructure,
(2) a single flyby of a low mass ($\lesssim 10^8$ M$_{\odot}$) has negligible
effect, (3) multiple flybys of subhalos derived from a subhalo mass function
between $10^7-10^{10}$ M$_{\odot}$ cause significant damage if deep within the
potential, (4) the effects of known perturbers (e.g. Sagittarius) should be
detectable and offer constraints on their initial mass. The sensitivity to the
populations of subhaloes suggests that we should be able to place an upper
limit on the Milky Way's subhalo mass function.
