Same fringe

var trees = [
    # 0..2 are same
    [ 'd', [ 'c', [ 'a', 'b', ], ], ],
    [ [ 'd', 'c' ], [ 'a', 'b' ] ],
    [ [ [ 'd', 'c', ], 'a', ], 'b', ],
    # and this one's different!
    [ [ [ [ [ [ 'a' ], 'b' ], 'c', ], 'd', ], 'e', ], 'f' ],
]

func get_tree_iterator(*rtrees) {
    var tree
    func {
        tree = rtrees.pop
        while (defined(tree) && tree.kind_of(Array)) {
            rtrees.append(tree[1])
            tree = tree[0]
        }
        return tree
    }
}

func cmp_fringe(a, b) {
    var ti1 = get_tree_iterator(a)
    var ti2 = get_tree_iterator(b)
    loop {
        var (L, R) = (ti1(), ti2())
         defined(L) &&  defined(R) && (L == R) && next
        !defined(L) && !defined(R) && return "Same"
        return "Different"
    }
}

for idx in ^(trees.end) {
    say ("tree[#{idx}] vs tree[#{idx+1}]: ",
           cmp_fringe(trees[idx], trees[idx+1]))
}

Output:

tree[0] vs tree[1]: Same
tree[1] vs tree[2]: Same
tree[2] vs tree[3]: Different