diff --git a/runtime.c b/runtime.c
index 6b6c4f44..df5d476b 100644
--- a/runtime.c
+++ b/runtime.c
@@ -2866,6 +2866,8 @@ object Cyc_thread_sleep(void *data, object timeout)
   return boolean_t;
 }
 
+// Copy given object to the heap, if it is from the stack.
+// This function is intended to be called directly from application code
 object copy2heap(void *data, object obj) 
 {
   char stack_pos;
diff --git a/srfi/18.sld b/srfi/18.sld
index 8fd37b75..8a5234a5 100644
--- a/srfi/18.sld
+++ b/srfi/18.sld
@@ -10,6 +10,9 @@
     thread-yield!
 ;    thread-terminate!
     ; For now, these are built-ins. No need for them here: make-mutex mutex-lock! mutex-unlock!
+    ;; TODO: consolidate built-ins, and convert them from primitives to FFI functions
+    ;;       in this module.
+    ;; Non-standard functions:
     ->heap
   )
   (begin
@@ -46,6 +49,11 @@
     (define (thread-yield!) (thread-sleep! 1))
 ;    (define (thread-terminate!) (Cyc-end-thread!))
     ;; TODO: thread-join!
+
+    ;; Take a single object and if it is on the stack, return a copy
+    ;; of it that is allocated on the heap. NOTE the original object
+    ;; will still live on the stack, and will eventually be moved
+    ;; itself to the heap if it is referenced during minor GC.
     (define-c ->heap
       "(void *data, int argc, closure _, object k, object obj)"
       " object heap_obj = copy2heap(data, obj);