I was porting some C# code and got this error: BCE0011: An error occurred during the execution of the step 'Boo.Lang.Compiler.Steps.EmitAssembly': 'Declaration referenced in a method implementation can not be a final method. Type: MyCollection. Assembly: run, Version=0.0.0.0.'. I distilled the problem down to 4 lines: import System import System.Collections class MyCollection(CollectionBase): def GetEnumerator() as IEnumerator: return null I tried adding "IEnumerable." to the def: def IEnumerable.GetEnumerator() as IEnumerator: which gave: problem05.boo(4,9): BCE0090: Derived method 'MyCollection.GetEnumerator' can not reduce the accessiblity of 'System.Collections.CollectionBase.GetEnumerator' from 'public' to 'private'. I added "public" to the def: public def IEnumerable.GetEnumerator() as IEnumerator: which gave: problem05.boo(4,16): BCE0116: Explicit member implementation for 'IEnumerable.GetEnumerator' must not declare any modifiers. I then peeked at Set.boo from the source and saw that it inherited (ICollection, IEnumerable). When I changed MyCollection to do the same (and took out CollectionBase) the compilation errors went away (other than missing abstract methods which I added and then successfully compiled). Has any one derived from CollectionBase in boo? Is this a bug in the compiler? -Chuck