Related to 43. The changes from 43 are the switch from an embedded
disjunction, and the fact that the disjunction is open. This means
that the disjunction is not simplified to a single branch, unlike 43.

See also 52.

-- a.cue --
DefA: A: "a"
DefB: B: "b"

First: {
	B: "b"
	C: "c"
}

First: {
	B: "b"
}

First: DefA | DefB
-- out/trim --
== a.cue
DefA: A: "a"
DefB: B: "b"

First: {
	B: "b"
	C: "c"
}

First: {
	B: "b"
}

First: DefA | DefB
