Resolve #2625 (#2627)

src/prelude/array.ts

@@ -0,0 +1,7 @@
+export function countIf<T>(f: (x: T) => boolean, xs: T[]): number {
+	return xs.filter(f).length;
+}
+
+export function count<T>(x: T, xs: T[]): number {
+	return countIf(y => x === y, xs);
+}