Boolean type based on existence

[hellcp]

Some APIs define the true/false state based on existence of the node.

So for example:

<root>
  <option/>
</root>

would evaluate option to be true
while:

<root>
</root>

would evaluate option to be false (or nil, in any case falsy).

This can't be done with the current Boolean type, and implementing it on top would break the compatibility, so it would need to be a new type.

[mvz]

Thanks for reporting this @hellcp, I'll see how this can be accommodated.