code: public Builder toBuilder() {
  		return builder(_version, _creator)
  			.metadata(_metadata)
  			.wayPoints(_wayPoints)
  			.routes(_routes)
  			.tracks(_tracks);
  	}
docs: Convert the <em>immutable</em> GPX object into a <em>mutable</em>
  builder initialized with the current GPX values.
  
  @since 1.1
  
  @return a new track builder initialized with the values of {@code this}
  GPX object