The number of lines whose User Line state is to be toggled. If specified as 0 or omitted, 1 is assumed.
The TU command will toggle the User Line state of the lines within the range of the command.
Lines that formerly were User Lines (U lines) will become ordinary lines (V lines), and lines that formerly were ordinary lines (V lines) will become User Lines (U lines).
Lines that are U lines are marked by a | vertical bar in the "gap column" while ordinary lines (V lines) do not have this mark.
When the User Line state of a line is toggled, the presence or absence of the | vertical bar will be the opposite of what it was before.
It is possible to use the LINE primary command to apply TU to a range of lines.
It is also possible to map TU to a key to quickly toggle the User Line status of a single line.
Here is an example mapping to do that, which will keep the cursor on the current line when finished:
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