Composer & Parsedown - Class 'UserFrosting\\Parsedown' not found

I managed to install Parsedown using composer with

    "require": {
        ...
        "erusev/parsedown": "^1.6"
    },

and added the class path to the autoload section

"autoload": {
    "classmap" : [
        "controllers", "middleware", "models", "plugins", "vendor/erusev/parsedown"
    ]
}

But when I try to execute this line...

    $Parsedown = new Parsedown();

... I end up with this error:

Class 'UserFrosting\Parsedown' not found

Running php composer.phar dump-autoload did not help.

What am I missing here? Why Parsedown is expected under UserFrosting - UserFrosting\Parsedown?

Here is the full composer.json:

{
    "name": "userfrosting/UserFrosting",
    "type": "project",
    "description": "A secure, modern user management system for PHP.",
    "keywords": ["php user management", "usercake", "bootstrap"],
    "homepage": "https://github.com/userfrosting/UserFrosting",
    "license" : "MIT",
    "authors" : [
        {
            "name": "Alexander Weissman",
            "homepage": "https://alexanderweissman.com"
        }
    ],
    "require": {
        "birke/rememberme" : "1.0.4",    
        "illuminate/database" : "5.0.33",
        "league/csv": "8.1.*",
        "nikic/php-parser" : "~1",
        "php" : ">=5.4.0",
        "phpmailer/phpmailer" : "5.2.10",        
        "twig/twig" : "~1.0",
        "slim/slim" : "2.*",
        "slim/views" : "0.1.3",
        "userfrosting/fortress" : "1.*",
        "wikimedia/composer-merge-plugin": "~1",
        "components/highlightjs": "9.8.0",
        "aws/aws-sdk-php": "3.*",
        "erusev/parsedown": "^1.6"
    },
    "extra": {
        "merge-plugin": {
            "include": [
                "plugins/*/composer.json"
            ],
            "recurse": true,
            "replace": false,
            "merge-dev": true,
            "merge-extra": false
        }
    },
    "autoload": {
        "classmap" : [
            "controllers", "middleware", "models", "plugins", "vendor/erusev/parsedown"
        ]
    }
}

Looks like you are trying to execute this line of code $Parsedown = new Parsedown(); in a class with namespace UserFrosting.

Either add a use block at the top of your php file, like this: use Parsedown; (this should come after namespace declaration), or type a backslash before the class name when you are using it, like so: $Parsedown = new \Parsedown();. The latter will start looking for this class in a root namespace.

You don't need to add this class to your autoload classmap section of composer.json file. If a package is pulled by composer, the composer will automatically add everything to autoloader after running dump-autoload.