Where can I find the definition for #copyForRealMemento?

It appears that this method should live in the object (or superclass) that is to be Magritte rendered.. I can't find it in my Squeak image but it is referenced by #pull as seen below.. Up to now I thought I was using the RealMemento class as specified by #mementoClass but just found that it needs to be defined on the instance side not the class side so my #mementoClass method was not being called until now.. Unfortunately now I get the DNU as indicated above.. MARealMemento>>pull "Utitlity method to copies the model into a cache populating nil values with the defaults specified by the descriptions" | result | result := self model copyForRealMemento. result description do: [ :each | (result readUsing: each) ifNil: [ result update: each default using: each ]. ]. ^ result