diff --git a/rotate.sh b/rotate.sh
index 103a29f..d05d52c 100755
--- a/rotate.sh
+++ b/rotate.sh
@@ -1,121 +1,124 @@
 #!/bin/bash
 ###
 # Stupid script to rotate a backup
 #
-# Author: Valerio B.
-# Date:   Wed 4 Ago 2020
+# Author:  Valerio B.
+# Date:    Wed 4 Ago 2020
+# License: CC 0 - public domain
 ##
 
 # do not proceed in case of errors
 set -e
 
 # current directory
 MYDIR="$(dirname "$(realpath "$0")")"
 
 # include all the stuff and useful functions
 . "$MYDIR"/bootstrap.sh
 
 # arguments
 place="$1"
 days="$2"
 max="$3"
+
+# expected file containing last timestamp
 last_timestamp_file="$place.timestamp"
 
 # current timestamp
 current_timestamp=$(date +%s)
 
 # show usage
 function show_help_rotate() {
 	echo "USAGE"
 	echo " $0 PATH DAYS MAX_ROTATIONS"
 	echo "EXAMPLE"
 	echo " $0 /home/backups 1 30"
 }
 
 # all the arguments must exist (just check the last one)
 if [ -z "$max" ]; then
 
 	echo "Bad usage"
 	show_help_rotate
 
 	exit 1
 fi
 
 # the place to be rotated must exist
 if [ ! -e "$place" ]; then
 	error "unexisting directory '$place'"
 
 	exit 2
 fi
 
 # validate max parameter
 if [ "$max" -lt 2 ]; then
 
 	echo "The MAX parameter must be greater than 1"
 
 	show_help_rotate
 
 	exit 3
 fi
 
 # check if the last timestamp was writed
 if [ -f "$last_timestamp_file" ]; then
 
 	# check the timestamp saved in the file
 	timestamp=$(<"$last_timestamp_file")
 	if [ "$timestamp" -lt 1000 ]; then
 		echo "bad format in file $last_timestamp_file"
 		exit
 	fi
 
 	# seconds spent from the last rotation
 	diff_seconds=$(expr "$current_timestamp" - "$timestamp")
 
 	# expected seconds from the last rotation before continuing
 	# NOTE: leave the star escaped to avoid syntax error in expr
 	expected_seconds=$(expr "$days" "*" 86400)
 
 	# check if it's not passed enought time
 	if [ "$diff_seconds" -lt "$expected_seconds" ]; then
 		warn "Doing nothing: last rotation was executed $diff_seconds seconds ago (expected at least $expected_seconds)"
 		exit
 	fi
 fi
 
 # save the last timestamp before rotating everything
 # this will avoid even parallel rotations
 echo $(date +%s) > "$last_timestamp_file"
 
 # eventually drop the last backup step
 # if it does not exist, don't care
 max_path="$place.$max"
 drop "$max_path"
 
 # shift all the backups
 after="$max"
 while [[ "$after" -gt 1 ]]; do
 
-	before=$(expr "$after - 1")
+	before=$(expr "$after" - 1)
 
 	# do not process the root directory for no reason in the world if you type that by mistake
 	# the --preserve-root is already implicit but... let's be sure! asd
 
 	before_path="$place.$before"
 	after_path="$place.$after"
 
 	# the source must exist. asd
 	if [ -e "$before_path" ]; then
 
 		# the trailing slash means: copy files and not just the directory
 		move "$before_path/" "$after_path"
 	fi
 
 	# next
 	after="$before"
 done
 
 # at the end, move the base forward
 # the trailing slash means: copy files and not just the directory
 copy "$place/" "$place.1"
 
 # now you are ready to overwrite "$place"