Updated hw6 to a newer version

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

hw6/hw4programs/lib15.oat

@@ -0,0 +1,13 @@
+string sub (string str, int start, int len) {
+  var arr = array_of_string(str);
+  var r = new int[len];
+  for (var i = 0; i < len; i = i + 1;) {
+      r[i] = arr[i+start];
+  }
+  return string_of_array (r);
+}
+
+int program (int argc, string []  argv) {
+  print_string (sub(argv[1], 3, 5));
+  return 0;
+}