Attribute/value pairs from the command line are matched against the attributes associated with certain processing instructions in the document. The instructions are <?xmlif if?> and its inverse <?xmlif if not?>, <?xmlif elif?> and its inverse <?xmlif elif not?>, <?xmlif else?>, and <?xmlif fi?>.
Argument/value pairs given on the command line are checked against the value of corresponding attributes in the conditional processing instructions. An `attribute match' happens if an attribute occurs in both the command-line arguments and the tag, and the values match. An `attribute mismatch' happens if an attribute occurs in both the command-line arguments and the tag, but the values do not match.
Spans between <?xmlif if?> or <?xmlif elif?> and the next conditional processing instruction at the same nesting level are passed through unaltered if there is at least one attribute match and no attribute mismatch; spans between <?xmlif if not?> and <?xmlif elif not?> and the next conditional processing instruction are passed otherwise. Spans between <?xmlif else?> and the next conditional-processing tag are passed through only if no previous span at the same level has been passed through. <?xmlif if?> and <?xmlif fi?> (and their `not' variants) change the current nesting level; <?xmlif else?> and <?xmlif elif?> do not.
All these processing instructions will be removed from the output produced. Aside from the conditionalization, all other input is passed through untouched; in particular, entity references are not resolved.
Value matching is by string equality, except that "|" in an attribute value is interpreted as an alternation character. Thus, saying foo='red|blue' on the command line enables conditions red and blue. Saying color='black|white' in a tag matches command-line conditions color='black' and color='white'.
Here is an example:
Always issue this text. <?xmlif if condition='html'?> Issue this text if 'condition=html' is given on the command line. <?xmlif elif condition='pdf|ps'?> Issue this text if 'condition=pdf' or 'condition=ps' is given on the command line. <?xmlif else?> Otherwise issue this text. <?xmlif fi?> Always issue this text.
Eric S. Raymond <esr@snark.thyrsus.com>