    "The first of the given values (usually a comprehension),
     if any."
    shared Absent|Value first<Value,Absent>(Iterable<Value,Absent> values)
            given Absent satisfies Null {
        Value? first;
        if (!is Finished next = values.iterator().next()) {
            first = next;
        }
        else {
            first = null;
        }
        assert (is Absent|Value first);
        return first;
    }