As it says here, we should generalize the notion of a Step taking over FlowNode creation. =)

Currently, if a Step doesn't have a block as an argument, it can't execute another Step within it. There are exceptions to this - as of this moment, parallel and load - but they're explicitly whitelisted in via a variable named 'hack', which...yeah. So I'd like to try to figure out how to do this in a more general case, because then some interesting possibilities in terms of composition show up.