## Question 4 [10 marks]
Recall the definition of the
“ProofLayout*
type constructor:
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infixr o :=: – the fixity and priority of the operator
data ProofLayout a = QED |
a :=: ProofLayout a deriving Show
Consider the definitions:
**’haskell
= 1+length xs
map f (xixs)
=+x：map f xs
Give a value
that proves the thßoren:
“: a -> b (length = length . (map f))
**Hintx* Use structural induction over lists.
proof4 :: (a
-> ProofLayout Int
= undefined
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