docs: public TrajectoryEnvelope getGroundEnvelope(long time) {
  		if (this.getTemporalVariable().getEST() > time || this.getTemporalVariable().getEET() < time) return null;
  		if (!this.hasSubEnvelopes()) return this;
  		for (TrajectoryEnvelope te : this.getGroundEnvelopes()) {
  			if (te.getTemporalVariable().getEST() <= time) {
  				if (te.getTemporalVariable().getEET() >= time) {
  					return te;
  				}
  			}
  		}
  		return null;
  	}