array of objects (string maps)
all keys of all the objects in the observed order.
import mir.test: should; auto o1 = ["a", "b"].StringMap!int([8, 8]); auto o2 = ["b", "c"].StringMap!int([8, 8]); auto o3 = ["c", "d"].StringMap!int([8, 8]); [o1, o2, o3].inclusiveHeader.should = ["a", "b", "c", "d"]; [o3, o2, o1].inclusiveHeader.should = ["c", "d", "b", "a"];