Fixtype of transactions.
This is a tagged union type, introduced by fty::deftagsum.
We model three kinds of transactions: bond a validator, unbond a validator, and something else that does not bond or unbond validators; we leave the details of third kind open, via a wrapper of any ACL2 value.
We do not model stake, so the bonding and unbonding transactions only contain an address (of the validator bonded or unbonded).