src/Collection/Collection.php
public function reduce(Closure $fn, $initial = null)
    {
        return array_reduce($this->elements, $fn, $initial);
    }
public function reduce(Closure $fn, $initial = null)
    {
        return array_reduce($this->elements, $fn, $initial);
    }
Reduce collection to a single value

@param Closure $fn
@param null $initial
@return mixed