indent versus previous line

Hi,


I got a file that contains those lines :


\s\s\s\s\sFOO
BAR1
BAR2
END_OF_BLOCK
\s\s\s\s\sFOO2
BAR1
BAR2
END_OF_BLOCK
\s\s\s\s\sFOO3
BAR1
BAR2
END_OF_BLOCK


I would like to indent all sublines of each BLOCK to be like that :

\s\s\s\s\sFOO
BAR1
BAR2
END_OF_BLOCK
\s\s\s\s\sFOO2
BAR1
BAR2
END_OF_BLOCK
\s\s\s\s\sFOO3
BAR1
BAR2
END_OF_BLOCK



My file contains 500000 lines so it has to be done automatically

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