Fixtype of transactions.
This is a tagged union type, introduced by fty::deftagsum.
Our model treats transactions as mostly opaque. Certain transactions bond and unbond validators, which is what makes the validator committee dynamic. So 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 third kind open.
We do not model stake, so the bonding and unbonding transactions only contain an address (of the validator bonded or unbonded).
We do not model solutions and ratifications explicitly. The third kind of transactions in our model can be thought of as including those as well.