Documentation:OCLOperators addReturnIndexOf0

The AddReturnIndexOf0 works like a normal Add - but returns the zero-based position of the object added.

This is particularly useful when dealing with innerlink objects - you add an object of Class2 to Class1.Class2s - and that results in a link object creation of Class3. You can access the object of Class3 with an expression like:

let x=self.Class2s.addReturnIndexOf0(Class2.Create) in
(
  self.Class3.at0(x).Attribute1:='Yes'
)

Or even more compact:

self.Class3.at0(self.Class2s.addReturnIndexOf0(Class2.Create)).Attribute1:='Yes2'