The document discusses representing instance variables in Pharo as first-class objects called slots. It describes how slots can be composed of a storage slot and decorator slots to combine behaviors. This solves the problem of wanting to combine certain slot types, like making a observable slot that also has a default value, without exploding the number of possible slot combinations. The proposed solution is a composed slot that has one storage slot and any number of decorator slots. Decorators modify behavior before and after reads and writes, while wrappers wrap the underlying slot. Most wrappers can be implemented as decorators instead to simplify composition. Status is work in progress to reimplement all existing slots to work with this composable model.