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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def iter_leaves(self, is_leaf_fn=None):
 """ Returns an iterator over the leaves under this node."""

        for n in self.traverse(strategy="preorder", is_leaf_fn=is_leaf_fn):
            if not is_leaf_fn:
                if n.is_leaf():
                    yield n
            else:
                if is_leaf_fn(n):
                    yield n