2.
(i) ('u Tree ->  'et -> 'u Tree -> 'u Tree) -> 'et Tree -> 'u Tree

(ii)
   tf : 'a -> 'b -> 'c Tree                  from the heading
   'a = 'c Tree -> 'd -> 'c Tree -> 'c Tree  from g(tf g L)...
   'b = 'd Tree  
so
   tf : ('c Tree -> 'd -> 'c Tree -> 'c Tree) -> 'd Tree -> 'c Tree

(iii) fun mirror t = tf (fn L => fn e => fn R => Fork(R,e,L)) t;
or similar

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   [[not unrelated to assignment-2, 2006]]