18 lines
530 B
Text
18 lines
530 B
Text
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import data.vector
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open nat
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namespace vector
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definition diag {A} {n} : vector (vector A n) n → vector A n :=
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nat.rec_on n
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(λv, nil)
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(λn₁ (r : vector (vector A n₁) n₁ → vector A n₁) (v : vector (vector A (succ n₁)) (succ n₁)),
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let h₁ : A := head (head v) in
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let t₁ : vector (vector A n₁) n₁ := map tail (tail v) in
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h₁ :: r t₁)
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example : diag ((1 :: 2 :: 3 :: nil) :: (4 :: 5 :: 6 :: nil) :: (7 :: 8 :: 9 :: nil) :: nil) = 1 :: 5 :: 9 :: nil :=
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rfl
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end vector
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