3f548edcd7
When updating Std, be careful that not every lemma has been upstreamed, so we need to be careful to only delete things that have already been declared.
18 lines
373 B
Text
18 lines
373 B
Text
mutual
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def isEven : Nat → Bool
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| 0 => true
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| n+1 => isOdd n
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decreasing_by
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sorry
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def isOdd : Nat → Bool
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| 0 => false
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| n+1 => isEven n
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decreasing_by
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sorry
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end
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theorem isEven_double (x : Nat) : isEven (2 * x) = true := by
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induction x with
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| zero => simp [isEven]
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| succ x ih => simp [Nat.mul_succ, Nat.add_succ, isEven, isOdd, ih]
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