added debug output to Backup()

calc.go

@@ -393,6 +393,10 @@ func (c *Calc) DoFuncall(funcname string) error {
 		return R.Err
 	}
 
+	// don't forget to backup!
+	c.stack.Backup()
+
+	// "pop"
 	if batch {
 		// get rid of stack
 		c.stack.Clear()

calc_test.go

@@ -236,6 +236,11 @@ func TestCalc(t *testing.T) {
 			exp:   2,
 			batch: true,
 		},
+		{
+			name: "undo",
+			cmd:  `4 4 + undo *`,
+			exp:  16,
+		},
 	}
 
 	for _, tt := range tests {

stack.go

@@ -198,10 +198,14 @@ func (s *Stack) Backup() {
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
 
+	s.Debug(fmt.Sprintf("backing up %d items from rev %d",
+		s.linklist.Len(), s.rev))
+
 	s.backup = list.List{}
 	for e := s.linklist.Front(); e != nil; e = e.Next() {
 		s.backup.PushBack(e.Value.(float64))
 	}
+
 	s.backuprev = s.rev
 }