basic rpn calculator with assignments

rpn.bash

set -f
IFS=

rpn () {
    for token do
        case $token in
            +|-|\*|/|%|\*\*) (( stack[-2] = stack[-2] $token stack[-1] )); unset stack[-1] ;;
            =) (( ${stack[-2]} = stack[-1] )); unset stack[-1] stack[-1] ;;
            *) stack+=($token) ;;
        esac
    done
    echo $* ...... ${stack[-1]}
}
rpn 3 4 2 * 1 5 - 2 3 ** ** / +
rpn 3 4 2 * +
rpn 1 5 -
rpn 3 2 **
rpn 2 2 + a 3 2 ** = a + a +
declare -p a

$ bash rpn.bash 
3 4 2 * 1 5 - 2 3 ** ** / + ...... 3
3 4 2 * + ...... 11
1 5 - ...... -4
3 2 ** ...... 9
2 2 + a 3 2 ** = a + a + ...... 22
declare -- a="9"```