Derived attributes & associations are read-only, but you can also have writable associations that update other attributes or links when they are themselves set. Read here Derived settable attributes about how that works on attributes.

OCL for reading Class2selection

self.Class2

EAL for the writing Class2Selection

self.Class2 := vInputParameter;
self.SelectionLog := self.SelectionLog + DateTime.Now.asString + ': '+ vInputParameter.Name + '\r\n'

Here is the sample model to be loaded into the designer: File:DerivedSettableExample.modlr