Default = > false
E:Type, t:Tree(E). is_leaf(t) 
| Who Cites is leaf? | |
| is_leaf | Def is_leaf(t) == Case(t) Case tree_leaf(l) = > true Default = > false |
| Thm* E:Type, t:Tree(E). is_leaf(t) | |
| case_default | Def Default = > body(value,value) == body |
| case_tree_leaf | Def Case tree_leaf(x) = > body(x) cont(x1,z) == InjCase(x1; x2. body(x2); _. cont(z,z)) |
| case | Def Case(value) body == body(value,value) |
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