Pop two elements, subtract, leave result on top of stack and in varname
USAGE:{
sub()
{
local arrayname=${1:?Array name required} varname=${2:-var}
Copy the array, $arrayname, to local array
eval "array=( \"\${$arrayname[@]}\" )"
Subtract values, store in array[1]
array[1]=$(( ${array[1]} - ${array[0]} ))
Store result in variable
printf -v "$varname" "%d" "${array[1]}"
unset array[0]
Copy array back to $arrayname
eval "$arrayname=( \"\$var\" \"\${array[@]}\" )"
}