src/Caller.php
public function on($middleware)
    {
        $this->middleware = $middleware ? (new Resolver)->resolveOrFail($middleware) : new Group;
        return $this;
    }
public function on($middleware)
    {
        $this->middleware = $middleware ? (new Resolver)->resolveOrFail($middleware) : new Group;
        return $this;
    }
$middleware setter.

@param $middleware
@return $this