  Conservation laws in the $1f_{7/2}$ shell model of $^{48}$Cr found in numeric
studies by Escuderos, Zamick and Bayman [A. Escuderos, L. Zamick, and B. F.
Bayman, arXiv:0506050 (2005)] and me [K. Neerg\aa rd, Phys. Rev. C \textbf{90},
014318 (2014)] are explained by symmetry under particle-hole conjugation and
the structure of the irreps of the symplectic group Sp(4). A generalization is
discussed.
