1 files changed, 11 insertions, 0 deletions
diff --git a/calc.go b/calc.go
index 47ece5b..67794aa 100644
--- a/calc.go
+++ b/calc.go
@@ -40,6 +40,17 @@ func (c *Calculator) swap() {
 	}
 }
 
+// dup duplicates the value at the bottom of the stack.
+func (c *Calculator) dup() {
+	v, err := c.stack.pop()
+	if err != nil {
+		// empty
+		return
+	}
+	c.stack.push(v)
+	c.stack.push(v)
+}
+
 // negate negates the number in the buffer, if any; or the bottom number on the
 // stack, if any.
 func (c *Calculator) negate() {