src/IteratorPipeline/Traits/AggregationTrait.php
public function reduce(callable $callback, $initial = null)
    {
        return i\iterable_reduce($this->iterable, $callback, $initial);
    }
public function reduce(callable $callback, $initial = null)
    {
        return i\iterable_reduce($this->iterable, $callback, $initial);
    }
Reduce all elements to a single value using a callback.

@param callable  $callback
@param mixed     $initial
@return mixed