POSIXLY_CORRECT=1
x=2
x+=5 eval printf '"$x "'
echo "$x"

unset x
typeset -i x=2
x+=5 eval printf '"$x "'
echo "$x"

a=1
a+=4
echo $a

# idiotically, ksh93 makes these two cases differ (?)
a+=5 printenv a
a+=5 eval printf '"$a "'
echo $a