Index: lib/typical/bitop.cpp
==================================================================
--- lib/typical/bitop.cpp
+++ lib/typical/bitop.cpp
@@ -3,5 +3,11 @@
 	int c = 0;
 	for(; x; x>>=1)
 		c += x&1;
 	return c;
 }
+
+
+/* for every nonempty subset of S  : O(2^|S|)
+ for(int sub=S; sub; sub=(sub-1)&S) { ... }
+
+*/