Entry(Env* e, Bucket* b, Entry* p, const T& v) : env(e), bucket(b), next_env(0), prev(p), value(v) { }
Entry(Env* e, Bucket* b, Entry* p, Entry* p_env, const T& v) : env(e), bucket(b), next_env(p_env), prev(p), value(v) { }
    Entry(Env* e, Bucket* b, Entry* p, const T& v) : env(e), bucket(b), next_env(0), prev(p), value(v) { }