Resolve #2629 (#2630)

src/prelude/array.ts

@@ -5,3 +5,7 @@ export function countIf<T>(f: (x: T) => boolean, xs: T[]): number {
 export function count<T>(x: T, xs: T[]): number {
 	return countIf(y => x === y, xs);
 }
+
+export function intersperse<T>(sep: T, xs: T[]): T[] {
+	return [].concat(...xs.map(x => [sep, x])).slice(1);
+}