code: function sanitize (msg) {
    if (typeof msg !== 'object') return null
    if (typeof msg.value !== 'object') return null
    if (typeof msg.value.content !== 'object') return null
    if (typeof msg.value.timestamp !== 'number') return null
    if (typeof msg.value.type !== 'string') return null
    return msg
  }
docs: Either returns a well-formed user message, or null.