code: @Beta
      public Difference<C> minus(final Interval<C> that) {
          return new Difference(this.intersect(that.ahead()),
                                this.intersect(that.behind()));
      }
docs: Experimental method to find the difference between two intervals.
  @param that interval
  @return the difference of this and that
  @see #ahead()
  @see #behind()
  @see #intersect(org.nmdp.ngs.fca.Interval)
  @see <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html#removeAll-java.util.Collection-">
  java.util.Collection.removeAll</a>
  @see <a href="https://en.wikipedia.org/wiki/Complement_(set_theory)">
  complement</a>