can't figure out how to add animation to this.

Ok, I have a piece of javascript but i have no clue how to make the collapse and expand animated. Do you guys have any ideas on how i would do this?

function toggle_collapse(B, E) {
    if (!is_regexp) {
        return false
    }
    var D = fetch_object("collapseobj_" + B);
    var A = fetch_object("collapseimg_" + B);
    var C = fetch_object("collapsecel_" + B);
    if (!D) {
        if (A) {
            A.style.display = "none"
        }
        return false
    }
    if (D.style.display == "none" || "open" == E) {
        D.style.display = "";
        if (!E) {
            save_collapsed(B, false)
        }
        if (A) {
            img_re = new RegExp("_collapsed\\.gif$");
            A.src = A.src.replace(img_re, ".gif")
        }
        if (C) {
            cel_re = new RegExp("^(thead|tcat)(_collapsed)$");
            C.className = C.className.replace(cel_re, "$1")
        }
    } else {
        if (D.style.display != "none" || "closed" == E) {
            D.style.display = "none";
            if (!E) {
                save_collapsed(B, true)
            }
            if (A) {
                img_re = new RegExp("\\.gif$");
                A.src = A.src.replace(img_re, "_collapsed.gif")
            }
            if (C) {
                cel_re = new RegExp("^(thead|tcat)$");
                C.className = C.className.replace(cel_re, "$1_collapsed")
            }
        }
    }
    return false
}

Code (markup):

 

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