issue 1298

See also 46, 52 and 56.

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

First: {
	#DefA | #DefB
	B: "b"
	C: "c"
}

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

First: {
	#DefA | #DefB
	B: "b"
	C: "c"
}

First: {
	B: "b"
}
