function changeToTab(new_tab_id) {
    for (i=0; i<tabs.length; i++) {
        document.getElementById(tabs[i]).style.display = 'none';
        document.getElementById('tab_' + tabs[i]).className = 'tab tab_inactive';
    }
    
    document.getElementById(new_tab_id).style.display = '';
    document.getElementById('tab_'+new_tab_id).className = 'tab activetab';
}