PHP Fatal error:Class not found error

Question

I am following a tutorial for generating pnml files using petrinet framework. I used this link from github and downloaded the php code. However, when I tried to run it locally on my system, I am not able to run due to the following error:

Fatal error: Class 'ziguss\petrinet\Element' not found

This error come when I try to execute the following code (Place.php)

<?php

namespace ziguss\petrinet;

class Place extends Element
{
}

The Element class is as shown below:

<?php

namespace ziguss\petrinet;

abstract class Element
{
    use Nameable;

    protected $inputArcs = [];
    protected $outputArcs = [];

    public function getInputArcs()
    {
        return $this->inputArcs;
    }

    public function addInputArc(Arc $inputArc)
    {
        $this->inputArcs[] = $inputArc;

        return $this;
    }

    public function getOutputArcs()
    {
        return $this->outputArcs;
    }


    public function addOutputArc(Arc $outputArc)
    {
        $this->outputArcs[] = $outputArc;

        return $this;
    }
}

Can anyone help me to fix this error.

Thanks in advance!