code: func (m *mruNonceMap) Exists(nonce uint64) bool {
  	m.mtx.Lock()
  	_, exists := m.nonceMap[nonce]
  	m.mtx.Unlock()
  
  	return exists
  }
docs: // Exists returns whether or not the passed nonce is in the map.
  //
  // This function is safe for concurrent access.