MarkLine            <Alt L> or <Esc><B><L>
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    This command is used to mark a line Block.  Initially executing this
    command will mark the current cursor line.  As the cursor is moved,
    the Block will "grow" with it line by line.  Executing this command
    again will mark the other end of the Block at the current cursor
    line.  If MarkLine is executed in a file that already contains a
    Block, the Block is extended to the current cursor line.