{
  "workUnitId": "W-C02",
  "contractVersion": "1.0.0",
  "sourceCase": "DEV-A",
  "prompt": "Implement a pure Python class Queue with constructor events=None (except the TTL case: ttl, events=None), observable events, and enqueue. Use only Python builtins; no imports or introspection. Preserve a supplied sink object and propagate its pre-append RuntimeError; no file/network actions. Return exactly JSON with code string. Work is a bounded serial exercise, not production payment handling. Runtime has the following builtins: object,type,int,bool,float,str,dict,list,tuple,set,len,isinstance,all,any,enumerate,range,sorted,min,max,sum,zip,reversed,next,iter,abs,hash,frozenset,map,filter,super,repr,round,property,staticmethod,classmethod,ValueError,TypeError,KeyError,AttributeError,NotImplementedError,IndexError,AssertionError,StopIteration,RuntimeError,Exception,getattr,hasattr. Private fields and __slots__ are supported. Imports, global/nonlocal mutation, dynamic __getattr__/__getattribute__/__del__, runtime globals/code/frame/traceback introspection are not permitted; getattr/hasattr reject those attributes. Such policy violations are recorded separately from incorrect queue behavior.\nenqueue(request_id, cents). cents must be positive integer, bool rejected even when an existing integer payload compares equal. New request appends one dict event containing id=request_id and cents=cents, and returns queued. Same ID and amount returns duplicate without another event; differing amount raises ValueError. A failed append must not occupy the ID, so retry can succeed.",
  "clarification": "Serial only; supplied failures occur before the failing append changes the sink. No concurrent or live business semantics."
}