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diff --git a/src/Common/Helper.v b/src/Common/Helper.v
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index 0000000..292d011
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+Module Option.
+
+Definition default {T : Type} (x : T) (u : option T) : T :=
+  match u with
+  | Some y => y
+  | _ => x
+  end.
+
+Definition map {S : Type} {T : Type} (f : S -> T) (u : option S) : option T :=
+  match u with
+  | Some y => Some (f y)
+  | _ => None
+  end.
+
+Definition liftA2 {T : Type} (f : T -> T -> T) (a : option T) (b : option T) : option T :=
+  match a with
+  | Some x => map (f x) b
+  | _ => None
+  end.
+
+End Option.