rat

@echo off & setlocal enabledelayedexpansion
set f=a.txt
for /f "eol= tokens=1,2* delims=:" %%a in ('findstr /n .* %f%') do (
	if %%a==1 (echo.%%b>%f%) else (
		if %%a lss 4 (echo.%%b>>%f%) else (
			if %%a==4 (
				
				set s=%%b
				set s=!s:~,1!
				if !s!==[ (goto :eof) else echo.%%b>>%f%
			) else (
				if %%a gtr 6 (goto :eof) else echo.%%b>>%f%
			)
		)
	)
)
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