Added comment, clean up stub API

src/link.js

@@ -4,6 +4,7 @@ import jiggle from "./jiggle";
 /**
  * Modified link force algorithm
  * - simplify calculations for parameters locked for spring model
+ * - replace the use of links {} with loop. greatly reduce memory usage
  * - removed other unused functions
  * Alpha should be constant 1 for accurate simulation
  */
@@ -39,6 +40,7 @@ export default function(links) {
 
   function initialize() {
     if (!nodes) return;
+    // 0.5 to divide the force to two part for source and target node
     dataSizeFactor = 0.5/(nodes.length-1);
     initializeDistance();
   }
@@ -57,14 +59,6 @@ export default function(links) {
     initialize();
   };
 
-  force.links = function(_) {
-    return arguments.length ? (links = _, initialize(), force) : links;
-  };
-
-  force.id = function(_) {
-    return arguments.length ? (id = _, force) : id;
-  };
-
   force.iterations = function(_) {
     return arguments.length ? (iterations = +_, force) : iterations;
   };