wise, a system default is used: zero for numeric types, the empty string for strings, <code>false</code> for bools. For embedded messages, the default value is always the “default instance” or “prototype” of the message, which has none of its fields set. Calling the accessor to get the value of an optional (or required) field which has not been explicitly set always returns that field’s default value.</li>
  <li><code>repeated</code>: the field may be repeated any number of times (including zero).