Value that does’t exist in the xml

Hello!
Will you be so kind to help me again? The xml I’ve got:

<cae>CTS148258</cae>
<price>4370</price>
<model>Brina Nordico V-522</model>
<thorn>Yes</thorn>

and

<cae>162204</cae>
<price>2190</price>
<model>Snowpro</model>

The <thorn>Yes is for products with thorns, for products without thorns there must be <thorn>No, but the supplier does not put it in the xml.

How can I put the <thorn> attribute “No” for all products which do not have <thorn>Yes?