def []=(key, value)
    debug { '[]= ' << key.to_s }

    entry = get_locked_entry(key)
    begin
      return entry.value = value
    ensure
      entry.sync.unlock
    end
  end
def []=(key, value)
    debug { '[]= ' << key.to_s }

    entry = get_locked_entry(key)
    begin
      return entry.value = value
    ensure
      entry.sync.unlock
    end
  end
store new value in cache

 see also Cache#fetch_or_add