public function afferentCoupling(DependencyMap $map) : array
    {
        return $map->fromDependencies()->reduce([], function (array $afferent, Dependency $from) use ($map) {
            $afferent[$from->toString()] = $map->reduce(
                0,
                function (int $count, Dependency $fromOther, Dependency $to) use ($from): int {
                    return $from->equals($to)
                        ? $count + 1
                        : $count;
                }
            );
            return $afferent;
        });
    }