If true, when the tab key is pressed when several lines are selected, the selected lines are indented of one level instead of being replaced with a \t character.
Shift+Tab unindents the selection.
If the first or last line is not selected completely, it is also indented or unindented.
When the selection doesn't span several lines, the tab key always replaces the selection with a normal \t character.
whether to indent a block when tab is pressed.