Those fifos are accessible during %sh{...} blocks, the command fifo
executes commands written to it once the write end side is closed
(multiple open/write/close sequences are supported), the response
fifo is a simple helper fifo available to write response back to
the shell process
An example use of this feature is to request some list options
content from without being limited by the environment size:
```
%sh{
echo "echo -to-file $kak_response_fifo -quoting shell -- %opt{some_list}" > $kak_command_fifo
eval "set -- $(cat $kak_response_fifo)"
}
```
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