The problem
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-We've seen two solutions to the same fringe problem so far.
The problem, recall, is to take two trees and decide whether they have
the same leaves in the same order.
So `ta` and `tb` are different trees that have the same fringe, but
`ta` and `tc` are not.
+We've seen two solutions to the same fringe problem so far.
The simplest solution is to map each tree to a list of its leaves,
then compare the lists. But because we will have computed the entire
fringe before starting the comparison, if the fringes differ in an