###'template_vars'
is called before vars are passed to the template engine to render the page.
The registered function can add,change or remove variables passed to template.
-$b is the array of vars pased to 'replace_macros()' function.
+$b is an array with:
+
+ 'template' => filename of template
+ 'vars' => array of vars passed to template
+
Complete list of hook callbacks