10 lines
246 B
Text
10 lines
246 B
Text
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import data.list
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open list
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definition head {A : Type} : Π (l : list A), l ≠ nil → A,
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head nil h := absurd rfl h,
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head (a :: l) _ := a
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theorem head_cons {A : Type} (a : A) (l : list A) (h : a :: l ≠ nil) : head (a :: l) h = a :=
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rfl
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