code: public static function keyValMap($traversable, callable $keyFn, callable $valFn)
      {
          $map = [];
          foreach ($traversable as $item) {
              $map[$keyFn($item)] = $valFn($item);
          }
  
          return $map;
      }
docs: @param mixed[]|Traversable $traversable
  
  @return mixed[][]