CurrentCertificateHasExpired is used exclusively to check if the current issued certificate has actually expired rather than just nearing expiry.

Gatherer is used to gather data about a Certificate in order to evaluate its current readiness/state by applying policy functions to it.

A chain of PolicyFuncs to be evaluated in order.

A Func evaluates the given input data and decides whether a re-issuance is required, returning additional human readable information in the 'reason' and 'message' return parameters if so.