code: public Interval<C> coalesce(final Interval<C> that) {
          if(this.overlaps(that)) {
              return new Interval(this.dimension, this.range.span(that.range));
          }
          return NULL;
      }
docs: Find the coalesced result of two overlapping intervals.
  @param that interval
  @return coalesced interval or #NULL if none exists
  @see Range#span(com.google.common.collect.Range)