“In our implementation where everything happens at run-time, trait applications are memorized to support composite traits and multiple trait applications to the same target class. Each time a trait is applied, it gets validated against the trait tree already bound to the target class. This ensures operators, overriding rule and flattening property will remain valid in the new trait tree. Unlike Squeak trait support, a trait can be applied at any time, and also the way the trait applications are memorized would make relatively trivial to unapply traits at runtime.”

— Full Trait Support for Objective-C (or almost)