odd_occurrences_in_array() {
    local -n _a="$1"
    local -A _count
    local _v
    for _v in "${_a[@]}"; do
        if [[ -z "${_count[$_v]:-}" ]]; then
            _count[$_v]=1
        else
            unset '_count[$_v]'
        fi
    done
    for _v in "${!_count[@]}"; do
        echo "$_v"
        return
    done
}