The reference describes HopObject.forEach(), but bizarrely compared to
Spidermonkey's/Rhino's Array.forEach (though it may be only an English issue).
Regardless, HopObject.forEach doesn't seem to exist. (I got a TypeError when I
tried to use it on one of my HopObject-extending prototypes.)

It seems to me that HopObject.forEach should exist and should work as exactly
like Array.forEach as possible. Fortunately, Mozilla offers the code they use
to implement Array.forEach at
<http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference:Objects:Array:forEach>,
and it's easily modified to work with HopObjects, like so:

HopObject.prototype.forEach = function(fun /*, thisp*/)
{ // forEach prototype slightly modified from Mozilla
    var tmp;
    var len = this.count();
    if (typeof fun != "function") throw new TypeError();
    var thisp = arguments[1];
    for (var i = 0; i < len; i++) {
        if (tmp = this.get(i)) fun.call(thisp, tmp, i, this);
    }
};

This is in the Helma 1.6 release with an updated rhino.jar from zumbrunn to fix
the e4x attributes issue.