Needing explanations theories vs alg of ElasticPPcpp example

[AnggaSetiawan]

Dear All

I have read the instruction for developing ElasticPPcpp material from; http://opensees.berkeley.edu/AddingYourCode.pdf. To develop new material from this example, I need more detail about the algorithm and theory relation in the defining the strain, stress, and tangent of ElasticPPcpp. Are there any body can help me?
Thank you

Regards