This file is indexed.

/usr/lib/os-probes/mounted/efi/20microsoft is in os-prober 1.76~deb9u1.

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The actual contents of the file can be viewed below.

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#!/bin/sh
# Detects Microsoft bootloader on a EFI System Partition

. /usr/share/os-prober/common.sh

efi="$1"

found=
for microsoft in $(item_in_dir microsoft "$efi"); do
	for boot in $(item_in_dir boot "$efi/$microsoft"); do
		bcd=$(item_in_dir bcd "$efi/$microsoft/$boot")
		bootmgfw=$(item_in_dir bootmgfw.efi "$efi/$microsoft/$boot")
		if [ -n "$bcd" -a -n "$bootmgfw" ]; then
			long="Windows Boot Manager"
			short=Windows
			path="$microsoft/$boot/$bootmgfw"
			found=true
			break
		fi
	done
done


if [ -n "$found" ]; then
	label="$(count_next_label "$short")"
	result "${path}:${long}:${label}"
fi
exit 0