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This allows other plugins that hook e.g. accept-line to identify the
fancy-ctrl-z widget as the caller and take appropriate steps when
necessary.

See https://github.com/tarruda/zsh-autosuggestions/issues/82#issuecomment-181133379

Use Ctrl-Z to switch back to Vim
I frequently need to execute random command in my shell. To achieve it I pause
Vim by pressing Ctrl-z, type command and press fg<Enter> to switch back to Vim.

The fg part really hurts me. I just wanted to hit Ctrl-z once again to get back
to Vim.
I could not find a solution, so I developed one on my own that works wonderfully
with ZSH.

Source: http://sheerun.net/2014/03/21/how-to-boost-your-vim-productivity/