src/Viserio/Component/Container/Container.php
public function singleton(string $abstract, $concrete = null): void
    {
        $concrete = $concrete ?? $abstract;

        $this->bindSingleton($abstract, $concrete);
    }
public function singleton(string $abstract, $concrete = null): void
    {
        $concrete = $concrete ?? $abstract;

        $this->bindSingleton($abstract, $concrete);
    }
{@inheritdoc}