2. ls: Label List
x 
nil 
(x nil)2. ls: Label List
3. u: Label
4. v: Label List
5. x
v (x v) x [u / v] (x [u / v]) 
x:Label, ls:Label List. x ls (x ls)| 1 | 1. x: Label 2. ls: Label List x nil (x nil) |
| 2 | 1. x: Label 2. ls: Label List 3. u: Label 4. v: Label List 5. x v (x v) x [u / v] (x [u / v]) |
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