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); } export function intersperse<T>(sep: T, xs: T[]): T[] { return [].concat(...xs.map(x => [sep, x])).slice(1); }