+ * enhancement: Optional features (not enabled by default) allow the
+ use of signals for inter-thread synchronization to be reduced on certain
+ supported platforms (currently Linux, Solaris, and FreeBSD on x86 and
+ x86-64). Set (and :sb-thread :sb-safepoint :sb-thruption :sb-wtimer)
+ to test these experimental features. Known remaining bugs include minor
+ thread safety issues, less timely freeing of memory by GC, and certain
+ (not yet optimally low) runtime overhead. Thanks to Anton Kovalenko.