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The basics of simple mappings are explained in Vim's excellent user manual at :help 05.4. You want some key combination (Shift + Tab) to do what's already provided by Vim's Ctrl + D command, so you map <S-Tab> (using :help key-notation now) to <C-d>. As Vim is a mode-based editor, you need to consider the mode; :help i_CTRL-D is for insert mode. ...

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