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Marc,

it's not actually missing support for long or polynomial division - in your example code \def is in the list of blacklisted tex tags - see function tex_sanitize_formula($texexp) in filter/tex/lib.php and

$tex_blacklist = array(
        'include','command','loop','repeat','open','toks','output',
        'input','catcode','name','^^',
        '\def','\edef','\gdef','\xdef',
        '\every','\errhelp','\errorstopmode','\scrollmode','\nonstopmode',
        '\batchmode','\read','\write','csname','\newhelp','\uppercase',
        '\lowercase','\relax','\aftergroup',
        '\afterassignment','\expandafter','\noexpand','\special',
        '\let', '\futurelet','\else','\fi','\chardef','\makeatletter','\afterground',
       '\noexpand','\line','\mathcode','\item','\section','\mbox','\declarerobustcommand'
    );

This is necessary here for security reasons... not meaning that all of these TeX tags are vulnerable but allowing free use of them may cause some "unexpected issues".