If the current line has some non-whitespace characters it won't match and we'll
just return `'0'` as before.
-If the current line *does* match the regex (i.e. i
s it's empty or just
+If the current line *does* match the regex (i.e. i it's empty or just
whitespace) we return the string `'-1'`.
Earlier I said that a line's foldlevel can be zero or a positive integer, so