On sums of $\mathcal{P}$-free forms under misère play

Milley and Renault proved an interesting characterisation of invertible elements in the dead-ending universe: they are the games with no subpositions of outcome $\mathcal{P}$ (the ‘$\mathcal{P}$-free’ games). We generalise their approach to obtain a stronger result and show in particular that the set of $\mathcal{P}$-free blocking games is closed under addition, which yields that every $\mathcal{P}$-free blocking game is invertible modulo the blocking universe. This has consequences for the invertible subgroups of various other misère monoids.