docs: public TreeSet<TrajectoryEnvelope> getGroundEnvelopes() {
  		TreeSet<TrajectoryEnvelope> ret = new TreeSet<TrajectoryEnvelope>(new Comparator<TrajectoryEnvelope>() {
  			@Override
  			public int compare(TrajectoryEnvelope o1, TrajectoryEnvelope o2) {
  				Bounds o1b = new Bounds(o1.getTemporalVariable().getEST(),o1.getTemporalVariable().getEET());
  				Bounds o2b = new Bounds(o2.getTemporalVariable().getEST(),o2.getTemporalVariable().getEET());
  				if (o2b.min-o1b.min > 0) return -1;
  				else if (o2b.min-o1b.min == 0) return 0;
  				return 1;
  			}
  		});			
  		if (this.getSubEnvelopes() != null) {
  			for (TrajectoryEnvelope te : this.getSubEnvelopes()) {
  				ret.addAll(te.getGroundEnvelopes());
  			}
  		}
  		else ret.add(this);
  		return ret;
  	}