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Our yard has several medium/large black walnut trees. Unfortunately, black walnuts do not have low branches. On several of the trees the first branch is more than 20' off the ground. To make matters worse, few of the trees are well situated for a tree house. 

The best we could come up with are two trees about 16' apart. That's a big span, but one of the pair is a large tree (the other is medium).

The challenge is how to design a support that will:

1. span the distance between the trees
2. be flexible enough to deal with potential wind problems (lift on the platform/tree house; sway in the trees
3. be strong enough to support the tree house