function _addElement(context, element, key) {
    var map = context._entityMaps[key];
    if (map[element._ID] == null) {
        var library = context[key];
        var indexPool = context._indexPools[key];
        if (indexPool.length) map[element._ID] = indexPool.pop();
        else map[element._ID] = library.length;
        library[map[element._ID]] = element;
    }
}
Private helper method to store an element in a library array.

@method
@private
@param {Object} context Object in possesion of the element arrays.
@param {Object} element The body, force, or constraint to add.
@param {String} key Where to store the element.
@return {undefined} undefined