code: public Interval span(Interval other) {
          Objects.requireNonNull(other, "other");
          int cmpStart = start.compareTo(other.start);
          int cmpEnd = end.compareTo(other.end);
          Instant newStart = (cmpStart >= 0 ? other.start : start);
          Instant newEnd = (cmpEnd <= 0 ? other.end : end);
          return Interval.of(newStart, newEnd);
      }
docs: Calculates the smallest interval that encloses this interval and the specified interval.
  <p>
  The result of this method will {@linkplain #encloses(Interval) enclose}
  this interval and the specified interval.
  
  @param other  the other interval to check for, not null
  @return the interval that spans the two intervals