public static function groupBy($traversable, callable $keyFn)
{
self::invariant(is_array($traversable) || $traversable instanceof \Traversable, __METHOD__ . ' expects array or Traversable');
$grouped = [];
foreach ($traversable as $key => $value) {
$newKeys = (array) $keyFn($value, $key);
foreach ($newKeys as $key) {
$grouped[$key][] = $value;
}
}
return $grouped;
}