+|@exists@|string|Presence of subproperty|@["properties","exists","my_subproperty"]@|
+|@contains@|string, array of strings|Presence of one or more keys or array elements|@["storage_classes_desired", "contains", ["foo", "bar"]]@ (matches both @["foo", "bar"]@ and @["foo", "bar", "baz"]@)
+(note @[..., "contains", "foo"]@ is also accepted, and is equivalent to @[..., "contains", ["foo"]]@)|