Which preposition to use with optation

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Fifthly, having thus lessened the power of the censors, consuls, praetors, and tribunes, he by way of compensationa serio-comic compensation it must have seemed to his shrewd yet superstitious mindrestored the right of co-optation to the sacred colleges of augurs and pontiffs, and increased their numbers, thus multiplying harmless objects of rivalry analogous to the ribands and garters of modern courts.

Which preposition to use with  optation