Template:Allbooks category/parent
This template attempts to include this allbooks category in one of its parents.
Usage
{{Allbooks category/parent|parent=<parent>|subject=<subject>}}
where <subject> is the name of the subject page associated with this allbooks category (thus, Subject:<subject>), and <parent> is a parent name provided by that subject page (thus, it is the value of a parameter {{{{parent}}} or {{{parent2}}} passed to a call to {{Subject page}} on page Subject:<subject>).
This template then checks that the parent subject page exists (Subject:<parent>) and is correctly formatted for automatic extraction of parameter values, and that its associated allbooks category exists; if not, it calls {{Allbooks category/error}}. If all is well, it adds this allbooks category to the parent's allbooks category.