--- rpn/README.orig	2005-09-28 22:25:27.000000000 -0400
+++ rpn/README	2005-09-28 22:26:56.000000000 -0400
@@ -16,5 +16,6 @@
 .   Print the top element
 <   Swap the top two elements
 d   Drop the top element
+D   Duplicate the top element
 
 C   Clear the stack
--- rpn/rpn.c.orig	2005-09-28 22:25:33.000000000 -0400
+++ rpn/rpn.c	2005-09-28 22:26:27.000000000 -0400
@@ -59,6 +59,10 @@
 		case 'd':
 			pop();
 			break;
+		case 'D':
+			x = pop();
+			push(x); push(x);
+			break;
 		case 'C':
 			clear();
 			break;