-simulate coroutines (see [[coroutines and aborts]], and [[assignment
-8]]). In that solution, we pulled the zipper on the first tree until
-we found the next leaf, then stored the zipper structure in the
-mutable variable while we turned our attention to the other tree.
+simulate coroutines (see [[coroutines and aborts]], and
+[[assignment8]]). In that solution, we pulled the zipper on the first
+tree until we found the next leaf, then stored the zipper structure in
+a mutable variable while we turned our attention to the other tree.