diff -r 50ce3667c190 -r 9b71dead1219 progs/pow.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/progs/pow.scala	Fri Nov 06 04:54:41 2015 +0000
@@ -0,0 +1,20 @@
+def concat(A: Set[String], B: Set[String]) : Set[String] =
+  for (x <-A ; y <- B) yield x ++ y
+
+def pow(A: Set[String], n: Int) : Set[String] = n match {
+  case 0 => Set("")
+  case n => concat(A, pow(A, n- 1))
+}
+
+val A = Set("a", "b", "c", "d")
+pow(A, 4).size                            // -> 256
+
+val B = Set("a", "b", "c", "")
+pow(B, 4).size                            // -> 121
+
+val C = Set("a", "b", "")
+pow(C, 2)
+pow(C, 2).size                            // -> 7
+
+pow(C, 3)
+pow(C, 3).size                            // -> 15