[1.7.0] Fix bug in HTMLPurifier_Harness that causes certain aspects of $input to change after parsing

- Add makeLookup() convenience function to HTMLModule
- Relocate SGML exclusion comment
- Add preliminary Bdo module test

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1049 48356398-32a2-884e-a903-53898d9a118a

library/HTMLPurifier/ElementDef.php

@@ -83,6 +83,14 @@ class HTMLPurifier_ElementDef
     
     /**
      * Lookup table of tags excluded from all descendants of this tag.
+     * @note SGML permits exclusions for all descendants, but this is
+     *       not possible with DTDs or XML Schemas. W3C has elected to
+     *       use complicated compositions of content_models to simulate
+     *       exclusion for children, but we go the simpler, SGML-style
+     *       route of flat-out exclusions, which correctly apply to
+     *       all descendants and not just children. Note that the XHTML
+     *       Modularization Abstract Modules are blithely unaware of such
+     *       distinctions.
      * @public
      */
     var $excludes = array();