var old_field = false;
var field = false;

function str_replace ( search, replace, subject ) {
    if(!(replace instanceof Array)){
        replace=new Array(replace);
        if(search instanceof Array){
            while(search.length>replace.length){
                replace[replace.length]=replace[0];
            }
        }
    }

    if(!(search instanceof Array))search=new Array(search);
    while(search.length>replace.length){
        replace[replace.length]='';
    }

    if(subject instanceof Array){
        for(k in subject){
            subject[k]=str_replace(search,replace,subject[k]);
        }
        return subject;
    }

    for(var k=0; k<search.length; k++){
        var i = subject.indexOf(search[k]);
        while(i>-1){
            subject = subject.replace(search[k], replace[k]);
            i = subject.indexOf(search[k],i);
        }
    }

    return subject;

}

function sub_menu(id) {
    if (id) {field = document.getElementById('field'+id).innerHTML;
    document.getElementById('field'+id).innerHTML = '<strong>'+field+'</strong>';}
if (id == '2')
{document.getElementById("sub_menu").innerHTML = '<a href="#">Лексика</a> | <a href="#">Грамматика</a>';}
if (id == '1') {document.getElementById("sub_menu").innerHTML = '&nbsp;';}
if (old_field) {
field = str_replace('<strong>','',document.getElementById('field'+id).innerHTML);
field = str_replace('<strong>','',field);
document.getElementById('field'+id).innerHTML = field;
}
old_field = id;
}

   if (id) {field = document.getElementById('field'+id).innerHTML;
    document.getElementById('field'+id).innerHTML =  '<strong>'+field+'</strong>';}
.
.
.
if (old_field) {
field =  str_replace('<strong>','',document.getElementById('field'+id).innerHTML);
field = str_replace('<strong>','',field);
document.getElementById('field'+id).innerHTML = field;
}
old_field = id;
}