[рони]

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var id_old;

function bg_click(id) {
    if (id_old != id) {

        document.getElementById(id).style.backgroundColor = "#00FF00";
        if (id_old) document.getElementById(id_old).style.backgroundColor = "";
        id_old = id;
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}
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<a href="#" id="a1" onclick="bg_click(this.id);return false">111111</a>
<a href="#" id="a2" onclick="bg_click(this.id);return false">222222</a>
<a href="#" id="a3" onclick="bg_click(this.id);return false">333333</a>
<a href="#" id="a4" onclick="bg_click(this.id);return false">444444</a>