ArmDiff
data class ArmDiff(val changed: List<String>, val ignored: List<String>, val cochanging: List<String>)(source)
The attributes that differ between two arms. ArmDiff.changed holds every non-volatile one, sorted, and includes ArmDiff.cochanging, the members a base image swap is known to bring along; ArmDiff.ignored holds the volatile and noisy ones, which never count.