When using category tree, don't keep the tag when the top-level categoy is changed

[mattdm]

Right now, I have tags in the left-most box

• howto
• troubleshooting
• meta
• explanations

These are all mandatory tags. The problem is that when browsing to see what's available, if you click through the categories, you end up adding multiple top-level tags. I can see now that site newbies are going to categorize many questions as all of these top level options by accident.