-/* this is used from Lisp in toplevel.lisp, replacing an older
- * (sigsetmask 0) - we'd like to find out when the signal mask is
- * not 0 */
+/* At the toplevel repl we routinely call this function. The signal
+ * mask ought to be clear anyway most of the time, but may be non-zero
+ * if we were interrupted e.g. while waiting for a queue. */