code: public Interval<C> gap(final Interval<C> that) {
          if(this.before(that)) {
              return this.ahead().intersect(that.behind());
          }
          if(this.after(that)) {
              return this.behind().intersect(that.ahead());
          }
          return NULL;
      }
docs: Find the gap between two intervals.
  @param that interval
  @return the interval between this and that or {@link Interval#NULL} if
  none exists
  @see #ahead()
  @see #behind()
  @see #intersect(org.nmdp.ngs.fca.Interval)
  @see <a href="https://en.wikipedia.org/wiki/Commutative_property">
  commutative property</a>