class Base {}

class C1a extends Base with Mixin1a {}

class C1b extends Base with Mixin1b {}

class C2a extends Base with Mixin2a {}

class C2b extends Base with Mixin2a, Mixin2b {}

class Mixin1a extends Base {}

class Mixin1b extends Mixin1a {}

mixin Mixin2a on Base {}
mixin Mixin2b on Base, Mixin2a {}