You can use integer counters to model 13.
–
+
(
–
)
Start with 1 negative counter.
Then add 3 more negative counters.
Add.
–
+
(
–
)
=
ref_doc_title.
Jumping to level 1 of 1
Excellent!
Now entering the Challenge Zone—are you ready?
Now entering the Challenge Zone—are you ready?