In this documentation:
It says, "And tab=-1 is the tab to the right of the start tab (if there is one). "
My testing indicates that -1 is the tab to the left.
Also, suggest making “if there is one” clearer. It sounds like it refers to the start tab which may or may not exist. Probably should always say “main” to refer to the tab-window that has focus when the macro starts.
“if there is one” might be restated to say “if there are any tabs to the left.”
Thinking further, I think the sentence
"And tab=-1 is the tab to the right of the start tab (if there is one). "
should say instead
"tab=-1 is the first tab to the left, tab=-2 the second, and so on."
You may come up with better replacement text than I did.