Cut it down, root it out or try to save it?
If the trunk damage extends half way or more around the trunk, you might as well cut down the tree. If it extends less than 1/4 of the way around the trunk, there's a good chance that the tree might recover. The ants crawling into the wood, indicates that there probably is decay in the main trunk, and that these are carpenter ants feeding in the decayed area. I suspect that the damage is fairly widespread.
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