Coords.php:
Coords.php:
-Revised some comments.
UniSim.php:
-Added a few more vector arithmetic test cases.

file:a/Coords.php -> file:b/Coords.php
--- a/Coords.php
+++ b/Coords.php
@@ -1,8 +1,7 @@
 <?php 
-abstract class Coords {
-	// This is a stub that may contain functions or parameters that apply universally to all coordinate systems,
-	// but may never be instantiated itself.
-	
+abstract class Coords {	
+	// Vector math only actually makes sense in Cartesian, so we have to 
+	// convert there, and convert back.
 	public function vector_add($vector2) {
 		$class = get_class($this);
 		$vector1 = $this->convert("CartesianCoords");

file:a/UniSim.php -> file:b/UniSim.php
--- a/UniSim.php
+++ b/UniSim.php
@@ -4,15 +4,15 @@
 require_once("Coords.php");
 
 $universe = new SpaceNode("universe node", 0, new SphericalCoords(0,0,0), null);
+echo $universe->describe();
 
 $node1 = new SpaceNode("node1", 40, new LinearCoords(40), $universe);
-
-echo $universe->describe();
 
 $infra_list = $universe->get_infra_list();
 foreach ($infra_list as $infra) {
 	echo $infra->describe();
 }
+echo $universe->describe();
 
 $vector1 = new SphericalCoords(5, M_PI/4, M_PI/8);
 printf("Vector1: %s\n", $vector1->string());
@@ -20,7 +20,11 @@
 printf("Vector2: %s\n", $vector2->string());
 $vector3 = $vector1->vector_add($vector2);
 printf("Vector3 = Vector1 + Vector2: %s\n", $vector3->string());
-$vector4 = $vector1->vector_subtract($vector2);
-printf("Vector4 = Vector1 - Vector2: %s\n", $vector4->string());
+$vector4 = $vector2->vector_add($vector1);
+printf("Vector4 = Vector2 + Vector1: %s\n", $vector4->string());
+$vector5 = $vector1->vector_subtract($vector2);
+printf("Vector5 = Vector1 - Vector2: %s\n", $vector5->string());
+$vector6 = $vector2->vector_subtract($vector1);
+printf("Vector6 = Vector2 - Vector1: %s\n", $vector6->string());
 
 ?>