Fallback Semantics for Attributes

To determine the value of an attribute, the <attr> attributes are processed one by one in order of appearance within the parent tag. For each definition, all combinations of variables used within it are enumerated, and all non-fail values produced are collected into a set:

• If the set contains exactly one value, it is taken as the value of the attribute.
• If the set contains multiple values, and the <attr> tag has a join attribute specified, the values are concatenated using the value of the join attribute as a delimiter, and the resulting string is used as the value for the repository attribute.
• Otherwise, the definition fails, and the next definition in the sequence is processed. If there are no definitions left, the attribute is left unspecified.

This behavior provides a way to determine if the variable has a specific value in its value set. The following example sets the attribute to False if the first parameter can be undefined, otherwise True:

<attr name='Completed' switch-var='%1'>
  <undef value='False'/>
</attr>
<attr name='Completed' value='True'/>