Hi, if I file-out the category Tests-Traits-MOP to own *.st file and then try to load it into the Pharo Kernel, this test will fail because of this expression: self assert: (MOPTestClassA>>#c2) origin = Trait2. in this case it shows that the origin of MOPTestClassA>>#c2 is Trait3. The solution of this issue was to recompile the loaded classes. But it shows that the we miss some mechanism for preservation of system consistency when changing classes with traits. Cheers, -- Pavel