9 lines
248 B
Text
9 lines
248 B
Text
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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