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It's time to load the problemtype. Go to Data --> Problemtype --> Examples --> cmas2d_customlib. The first you can see is a window like this: 
This window helps you to generate a random 4 sided surface. For this example let's click Random surface and get an autogenerated surface. You can click continue and create your own surface. This is the surface I'll work with: 
After this, let's open the properties tree. Go to Data --> Data tree.
Interface definition
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