Allow excluding items with hook_xmlsitemap_element_alter()

Problem/Motivation

Say, I have 3 sitemaps and I want to remove specific items from those sitemaps based on very complex business logic. I try setting $element = NULL in hook_xmlsitemap_element_alter() but that causes problems because other hook implementations expect it to be an array.

Steps to reproduce

NA

Proposed resolution

Create a flag in &$element to have it excluded, say, $element['exclude'] = TRUE.

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD