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... iam iamque adfore obtestans vindictae properam, sive servitia armaret...

I note several examples w. the gen. in L. & S. s.v. "properus," including this passage: With gen. (Tacitean): Agrippina oblatae occasionis propera, quickly seizing the proffered occasion, Tac. A. 12, 66: irae, id. ib. 11, 26: vindictae, id. ib. 14, 7.—With inf. (Tacitean): quoquo facinore properus clarescere, Tac. A. 4, 52