7 lines
340 B
Text
7 lines
340 B
Text
theorem ex [Add α]
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(assoc : {a b c : α} → a + b + c = a + (b + c))
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(comm : {a b : α} → a + b = b + a)
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(f : α → α) (x y z : α) : f (x + (y + z)) = f (y + (x + z)) := by
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let leftAssoc {a b c : α} : a + (b + c) = b + (a + c) := by
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rw [← assoc, comm (a := a), assoc]
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simp [leftAssoc]
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