  In this note, we define the notion of a cactus set, and show that its
geometric realization is naturally an algebra over Voronov's cactus operad,
which is equivalent to the framed 2-dimensional little disks operad
$\mathcal{D}_2$. Using this, we show that the Hochschild cohomology of a
Poincar\'e algebra A is an algebra over (the chain complexes of)
$\mathcal{D}_2$.
