function append(a, b)
    {
        if (typeof b === "string")
            b = createTextNode(b);
        else if (b.shadowRoot)
            b = createTextNode(b.shadowRoot.lastChild.textContent);
        element.appendChild(b);
    }
@param {string} format
@param {?ArrayLike} substitutions
@param {?string} initialValue
@return {!Element}