This model represents the expression .
d
+
d
+
d
+
d
+
k
+
k
d
d
d
d
k
k
Which expression is equivalent to ?
d
+
d
+
d
+
d
+
k
+
k
4
d
+
2
k
4
d
+
k
6
+
d
+
k
5
d
+
k
ref_doc_title.
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