Which letter may precede the shield: $(x,0,1,6,3)$ is tree-safe iff $x\equiv1$ (mod 4) or $x\in\{7,16\}$

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Which letter may precede the shield: $(x,0,1,6,3)$ is tree-safe iff $x\equiv1$ (mod 4) or $x\in\{7,16\}$ · Prove2Me