comment the implementation

Author: SethTisue

src/main/scala/scala/collection/immutable/next/package.scala

@@ -21,10 +21,18 @@ package object next {
      * The result is a true cycle occupying only constant memory.
      */
     def cycle: LazyList[T] =
+      // case 1: the input is already known to be empty
+      // (the test can be changed to ll.knownIsEmpty when this code moves to stdlib)
       if (ll.knownSize == 0) LazyList.empty
+      // we don't want to force the input's empty-or-not status until we must.
+      // `LazyList.empty #:::` accomplishes that delay
       else LazyList.empty #::: {
+        // case 2: the input is later discovered to be empty
+        // (distinguishing this from case 3 isn't necessary for correctness, but we
+        // might as well avoid the lazy val path if we can)
         if (ll.isEmpty) LazyList.empty
         else {
+          // case 3: non-empty
           lazy val result: LazyList[T] = ll #::: result
           result
         }