# remove #pragma once:
grep -r --include=*.h -Fx '#pragma once' -l . \
| while read f; do
    hg="`realpath --relative-to=. "$f" | tr '/.[:lower:]' '__[:upper:]'`"
    sed -i -e "/#pragma once/ c \\
#ifndef $hg\\
#define $hg
$ a \\
\\
#endif  // $hg" "$f";
done