How to display a default value, if the variable is not set?

From Invenzzia wiki

Contrary to PHP, in OPT you can freely use undeclared variables, which are treated like having null values. However, sometimes we would like to display some other default value, if the variable is empty. There are several ways to do it in Open Power Template. The simplest one is to use firstof() function, which returns the first non-empty argument:

<p>This is a value: {firstof($variable, 'Default value')}</p>

You can provide more than two attributes to this function, so that the template could choose from more options:

<p>This is a value: {firstof($variable, $defaultCustomizedValue, 'Default value')}</p>

The second way is to use opt:content attribute:

<tr>
  <td>Cell name:</td>
  <td opt:content="$value">Default cell value</td>
</tr>