+
+ /* there are potentially two stacks for each thread: the main
+ * stack, which may contain Lisp pointers, and the alternate stack.
+ * We don't ever run Lisp code on the altstack, but it may
+ * host a sigcontext with lisp objects in it */
+
+ /* what we need to do: (1) find the stack pointer for the main
+ * stack; scavenge it (2) find the interrupt context on the
+ * alternate stack that might contain lisp values, and scavenge
+ * that */
+
+ /* we assume that none of the preceding applies to the thread that
+ * initiates GC. If you ever call GC from inside an altstack
+ * handler, you will lose. */