BLS: Not a blocker: Sign new key with removed members

[jeanphilippeD]

For Future reference.
We don't need to solve this now, I think.

Updating the keys

Whenever the DKG is complete and a new set of keys is generated, the Elders SHALL use their old keys to sign the new aggregate public key - in case of splits, both of the new public keys. This is a requirement of RFC 56. The signature shares SHALL be input into PARSEC, and only once enough shares reach consensus for the signature to be constructed, will the Elders discard their old BLS keys.

The signature along with the new section's public key SHALL be appended to the SectionProofChain in the shared state (see RFC 56).

Removed elder will generally not be faulty, they may be relocated or another older node joined in. How will these removed elder do "The signature shares SHALL be input into PARSEC"?