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1. If the update set contains the reserve currency and it rate isn't 1, the update is rejected.
2. If any new rate falls outside the range [10^{-6}, 10^6]; update is rejected.
3. If any new rate falls outside the range OldRate/2, 2 x OldRate; update is rejected.
4. if all new rates fall within a multiple of 1.0001 of old rates, then the update is accepted but not performed (i.e. all or none are accepted).

If the above validation checks all pass, then exchange rates are updated and rate-dependent cached values are recomputed.

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