code: 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;
  	}
docs: Get the ground {@link TrajectoryEnvelope} in which the robot will be
  at a given time. Returns <code>null</code> if the robot will not be in this
  {@link TrajectoryEnvelope} at the given time, or if the robot is not in any ground envelope
  at the given time (e.g., traveling between the end point of one ground envelope and
  the starting point of the next one).
  @param time The time at which to return the {@link TrajectoryEnvelope}.
  @return The ground {@link TrajectoryEnvelope} in which the robot will be
  at a given time.