Convoke only lets you select one creature at a time

When you select Convoke as a "special" cost, it only lets you select on creature, before returning to the remainder of the mana cost. Invoke, meanwhile, lets you select multiple artifacts before selecting OK. It would be convenient to be able to convoke multiple creatures at once. Maybe I'll look into it.

No file or test is identified in the issue. Start by comparing the Convoke and Invoke special-cost selection flows; done means selecting multiple creatures for Convoke before returning to the remaining mana cost.

Written by the indexing model from the issue text.