source: vendor/bash/3.1-p17/tests/appendop.tests

# basic cases
a=1
a+=4
echo $a

x=(1 2 3)
x+=(4 5 6)

echo ${x[@]}

x[4]+=1
echo ${x[@]}

# trickier cases

a+=5 printenv a
echo $a

# if the integer flag is set, ksh93 appears to do arithmetic += and evaluate
# old value as an arithmetic expression
a=
typeset -i a
a+=7
echo $a

b=4+1
typeset -i b
b+=37

echo $b

unset x
x=(1 2 3 4 5)

typeset -i x

x[4]+=7

echo ${x[@]}

unset x
typeset -i x

x=([0]=7+11)
echo ${x[@]}

unset x
x=(1 2 3 4 5)

typeset -i x

#x[4]=7+11

x=(1 2 3 4 [4]=7+11 )
echo ${x[@]}

x=( 1 2 [2]+=7 4 5 )
echo ${x[@]}

x+=( [3]+=9 [5]=9 )
echo ${x[@]}

unset a
a=1
export a+=4
printenv a
printenv a+

unset x
typeset -i x=4+5
echo $x

unset x
typeset x+=4
echo $x

typeset -i x+=5
echo $x

readonly x+=7
echo $x

x+=5