A mapping from grandpa set ID to the index of the most recent session for which its
members were responsible.
This is only used for validating equivocation proofs. An equivocation proof must
contains a key-ownership proof for a given session, therefore we need a way to tie
together sessions and GRANDPA set ids, i.e. we need to validate that a validator
was the owner of a given key on a given session, and what the active set ID was
during that session.
A mapping from grandpa set ID to the index of the most recent session for which its members were responsible.
This is only used for validating equivocation proofs. An equivocation proof must contains a key-ownership proof for a given session, therefore we need a way to tie together sessions and GRANDPA set ids, i.e. we need to validate that a validator was the owner of a given key on a given session, and what the active set ID was during that session.
TWOX-NOTE:
SetIdis not under user control.