State modificaiton anomaly (or mixin anomaly)

• Mixing "lock" class to bbuf.

  lock = interface { free, locked }
    new = method() replies(self{free})
    lock = method({free}l:self{locked})
    free = method({locked}l:self{free})
  end lock
  locki = class implements lock
    new = method() replies(self{free}) reply(self$[]!{free}) end new
    lock = method({free}l:self{locked}) l!{locked} end lock
    free = method({locked}l:self{free}) l!{free} end free
  end locki
  bbufl = interface extends bbuf and lock
    new = method(n:int) replies(self{empty.free})
    put = method({empty.free,mid.free}b:self{mid.free,full.free}, i:int)
    get = method({full.free,mid.free}b:self{mid.free,empty.free})
            replies(int)
  end bbufl