[Paul Stenning]

Overlining isn't a HTML styling option that can be applied with HTML tags, which is why there are no BB tags for it (the system translates the BB tags in square brackets to HTML tags for display).

The best approach in that case would be to decide on an alternative way for your post such as -CE or /CE, or use a colour, and state at the start that this method implies active low. Or type it in something that does support overlining etc and save as a PDF to attach here. Not ideal but that's all we can do here.