13 lines
286 B
Text
13 lines
286 B
Text
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import data.list
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open list
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variable {T : Type}
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theorem append.assoc : ∀ (s t u : list T), s ++ t ++ u = s ++ (t ++ u)
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| append.assoc nil t u := by esimp
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| append.assoc (a :: l) t u :=
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begin
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change a :: (l ++ t ++ u) = (a :: l) ++ (t ++ a),
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rewrite append.assoc
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end
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