// Detect browser quirks that we should be aware of.

function needsDynPrefix() {
    var span = document.createElement("span");
    span.innerHTML = "<script>alert('test');</script>";
    var scripts = span.getElementsByTagName("script");
    return scripts.length == 0;
}

var dynPrefix = needsDynPrefix() ? "<span style=\"display:none\">A</span>" : "";

// Function versions of operators

function not(x) { return !x; }
function neg(x) { return -x; }

function eq(x, y) { return x == y; }
function plus(x, y) { return x + y; }
function minus(x, y) { return x - y; }
function times(x, y) { return x * y; }
function div(x, y) { return x / y; }
function divInt(x, y) { var n = x / y; return n < 0 ? Math.ceil(n) : Math.floor(n); }
function mod(x, y) { return x % y; }
function modInt(x, y) { var n = x % y; return n < 0 ? Math.ceil(n) : Math.floor(n); }
function lt(x, y) { return x < y; }
function le(x, y) { return x <= y; }

// Characters

function isLower(c) { return c >= 'a' && c <= 'z'; }
function isUpper(c) { return c >= 'A' && c <= 'Z'; }
function isAlpha(c) { return isLower(c) || isUpper(c); }
function isDigit(c) { return c >= '0' && c <= '9'; }
function isAlnum(c) { return isAlpha(c) || isDigit(c); }
function isBlank(c) { return c == ' ' || c == '\t'; }
function isSpace(c) { return isBlank(c) || c == '\r' || c == '\n'; }
function isXdigit(c) { return isDigit(c) || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F'); }
function toLower(c) { return c.toLowerCase(); }
function toUpper(c) { return c.toUpperCase(); }


// Lists

function cons(v, ls) {
    return { next : ls, data : v };
}
function rev(ls) {
    var acc = null;
    for (; ls; ls = ls.next)
        acc = cons(ls.data, acc);
    return acc;
}
function concat(ls1, ls2) {
    var acc = ls2;
    ls1 = rev(ls1);
    for (; ls1; ls1 = ls1.next)
        acc = cons(ls1.data, acc);
    return acc;
}
function member(x, ls) {
    for (; ls; ls = ls.next)
        if (ls.data == x)
            return true;
    return false;
}
function remove(x, ls) {
    var acc = null;

    for (; ls; ls = ls.next)
        if (ls.data == x)
            return concat(acc, ls.next);
        else
            acc = cons(ls.data, acc);

    return ls;
}
function union(ls1, ls2) {
    var acc = ls2;

    for (; ls1; ls1 = ls1.next)
        if (!member(ls1.data, ls2))
            acc = cons(ls1.data, acc);

    return acc;
}
function length(ls) {
    var acc = 0;

    for (; ls; ls = ls.next)
        ++acc;

    return acc;
}


// Time

function showTime(tm) {
    var newDate = new Date();
    newDate.setTime(tm / 1000);
    return newDate.toUTCString();
}

function now() {
    return (new Date()).getTime() * 1000;
}

function diffInSeconds(tm1, tm2) {
    return Math.round((tm2 - tm1) / 1000000);
}

function toSeconds(tm) {
    return Math.round(tm / 1000000);
}

function addSeconds(tm, n) {
    return tm + n * 1000000;
}


// Error handling

function whine(msg) {
    alert(msg);
    throw msg;
}

function pf(loc) {
    throw ("Pattern match failure (" + loc + ")");
}

function runHandlers(kind, ls, arg) {
    if (ls == null)
        alert(kind + ": " + arg);
    for (; ls; ls = ls.next)
        try {
            exec({c:"a", f:{c:"a", f:ls.data, x:{c:"c", v:arg}}, x:{c:"c", v:null}});
        } catch (v) { }
}

var errorHandlers = null;

function onError(f) {
    errorHandlers = cons(f, errorHandlers);
}

function er(s) {
    runHandlers("Error", errorHandlers, s);
    throw {uw_error: s};
}

var failHandlers = null;

function onFail(f) {
    failHandlers = cons(f, failHandlers);
}

function doExn(v) {
    if (v == null || v.uw_error == null) {
        var s = (v == null ? "null" : v.message ? v.message : v.toString());
        if (v != null && v.fileName && v.lineNumber)
            s += " (" + v.fileName + ":" + v.lineNumber + ")";
        runHandlers("Fail", failHandlers, s);
    }
}

var disconnectHandlers = null;

function flift(f) {
    return {c: "c", v:{env:cons(f,null), body:{c:"v", n:1}}};
}

function onDisconnect(f) {
    disconnectHandlers = cons(flift(f), disconnectHandlers);
}

function discon() {
    runHandlers("Disconnect", disconnectHandlers, null);
}

var connectHandlers = null;

function onConnectFail(f) {
    connectHandlers = cons(flift(f), connectHandlers);
}

function conn() {
    runHandlers("Connect", connectHandlers, null);
}

var serverHandlers = null;

function onServerError(f) {
    serverHandlers = cons(f, serverHandlers);
}

function servErr(s) {
    window.setTimeout(function () { runHandlers("Server", serverHandlers, s); }, 0);
}


// Embedding closures in XML strings

function cs(f) {
    return {closure: f};
}

function isWeird(v) {
    return v.closure != null || v.cat1 != null;
}

function cat(s1, s2) {
    if (isWeird(s1) || isWeird(s2))
        return {cat1: s1, cat2: s2};
    else
        return s1 + s2;
}

var closures = [];
var freeClosures = null;

function newClosure(f) {
    var n;
    if (freeClosures == null) {
        n = closures.length;
    } else {
        n = freeClosures.data;
        freeClosures = freeClosures.next;
    }
    closures[n] = f;
    return n;
}

function freeClosure(n) {
    closures[n] = null;
    freeClosures = cons(n, freeClosures);
}

function cr(n) {
    return closures[n];
}

function flatten(cls, tr) {
    if (tr.cat1 != null)
        return flatten(cls, tr.cat1) + flatten(cls, tr.cat2);
    else if (tr.closure != null) {
        var cl = newClosure(tr.closure);
        cls.v = cons(cl, cls.v);
        return "cr(" + cl + ")";
    } else
        return tr;
}

function flattenLocal(s) {
    var cls = {v : null};
    var r = flatten(cls, s);
    for (cl = cls.v; cl != null; cl = cl.next)
        freeClosure(cl.data);
    return r;
}



// Dynamic tree management

function populate(node) {
    var s = node.signal;
    var oldSources = node.sources;
    try {
        var sr = execF(s, null);
        var newSources = sr._sources;

        for (var sp = oldSources; sp; sp = sp.next)
            if (!member(sp.data, newSources))
                sp.data.dyns = remove(node, sp.data.dyns);

        for (var sp = newSources; sp; sp = sp.next)
            if (!member(sp.data, oldSources))
                sp.data.dyns = cons(node, sp.data.dyns);

        node.sources = newSources;
        node.recreate(sr._data);
    } catch (v) {
        doExn(v);
    }
}

function sc(v) {
    return {data : v, dyns : null};
}
function sv(s, v) {
    if (s.data != v) {
        s.data = v;

        for (var ls = s.dyns; ls; ls = ls.next)
            if (!ls.dead)
                populate(ls.data);
    }
}
function sg(s) {
    return s.data;
}

function ss(s) {
    return {env:cons(s, null), body:{c:"r", l:
            cons({n:"sources", v:{c:"c", v:cons(s, null)}},
                 cons({n:"data", v:{c:"f", f:sg, a:cons({c:"v", n:1}, null)}}, null))}};
}
function sr(v) {
    return {env:null, body:{c:"c", v:{_sources : null, _data : v}}};
}
function sb(x,y) {
    return {env:cons(y,cons(x,null)),
            body:{c:"=",
                e1:{c:"a", f:{c:"v", n:2}, x:{c:"c", v:null}},
                e2:{c:"=",
                    e1:{c:"a",
                        f:{c:"a", f:{c:"v", n:2}, x:{c:".", r:{c:"v", n:0}, f:"data"}},
                        x:{c:"c", v:null}},
                    e2:{c:"r", l:cons(
                                      {n:"sources", v:{c:"f", f:union, a:cons({c:".", r:{c:"v", n:1}, f:"sources"},
                                                                              cons({c:".", r:{c:"v", n:0}, f:"sources"}, null))}},
                                      cons({n:"data", v:{c:".", r:{c:"v", n:0}, f:"data"}}, null))}}}};
}
function scur(s) {
    return execF(s, null)._data;
}

function lastParent() {
    var pos = document.body;

    while (pos.lastChild && pos.lastChild.nodeType == 1)
        pos = pos.lastChild;

    pos = pos.parentNode;

    return pos;
}

function parent() {
    return thisScript ? thisScript.parentNode : lastParent();
}

function addNode(node) {
    if (thisScript)
        thisScript.parentNode.replaceChild(node, thisScript);
    else
        lastParent().appendChild(node);
}

var thisScript = null;

function runScripts(node) {
    if (node.getElementsByTagName) {
        var savedScript = thisScript;

        var scripts = node.getElementsByTagName("script"), scriptsCopy = [];
        var len = scripts.length;
        for (var i = 0; i < len; ++i)
            scriptsCopy[i] = scripts[i];
        for (var i = 0; i < len; ++i) {
            thisScript = scriptsCopy[i];

            try {
                eval(thisScript.text);
            } catch (v) {
                doExn(v);
            }
            if (thisScript.parentNode)
                thisScript.parentNode.removeChild(thisScript);
        }

        thisScript = savedScript;
    }
}


// Dynamic tree entry points

function killScript(scr) {
    scr.dead = true;
    for (var ls = scr.sources; ls; ls = ls.next)
        ls.data.dyns = remove(scr, ls.data.dyns);
    for (var ls = scr.closures; ls; ls = ls.next)
        freeClosure(ls.data);
}

function dyn(pnode, s) {
    var x = document.createElement("script");
    x.dead = false;
    x.signal = s;
    x.sources = null;
    x.closures = null;

    var firstChild = null;

    x.recreate = function(v) {
        for (var ls = x.closures; ls; ls = ls.next)
            freeClosure(ls.data);

        var next;
        for (var child = firstChild; child && child != x; child = next) {
            next = child.nextSibling;

            killScript(child);
            if (child.getElementsByTagName) {
                var arr = child.getElementsByTagName("script");
                for (var i = 0; i < arr.length; ++i)
                    killScript(arr[i]);
            }

            if (child.parentNode)
                child.parentNode.removeChild(child);
        }

        var cls = {v : null};
        var html = flatten(cls, v);
        if (pnode != "table" && pnode != "tr")
            html = dynPrefix + html;
        x.closures = cls.v;

        if (pnode == "table") {
            var dummy = document.createElement("body");
            dummy.innerHTML = "<table>" + html + "</table>";
            runScripts(dummy);
            var table = x.parentNode;

            if (table) {
                firstChild = null;
                var tbody;

                var arr = dummy.getElementsByTagName("tbody");

                var tbody;
                if (arr.length > 0 && arr[0].parentNode == dummy.firstChild) {
                    tbody = arr[0];
                    var next;
                    for (var node = dummy.firstChild.firstChild; node; node = next) {
                        next = node.nextSibling;

                        if (node.tagName != "TBODY")
                            tbody.appendChild(node);
                    }
                } else
                    tbody = dummy.firstChild;

                var next;
                firstChild = document.createElement("script");
                table.insertBefore(firstChild, x);
                for (var node = tbody.firstChild; node; node = next) {
                    next = node.nextSibling;
                    table.insertBefore(node, x);
                }
            }
        } else if (pnode == "tr") {
            var dummy = document.createElement("body");
            dummy.innerHTML = "<table><tr>" + html + "</tr></table>";
            runScripts(dummy);
            var table = x.parentNode;

            if (table) {
                var arr = dummy.getElementsByTagName("tr");
                firstChild = null;
                var tr;
                if (arr.length > 0 && table != null)
                    tr = arr[0];
                else
                    tr = dummy.firstChild;

                var next;
                firstChild = document.createElement("script");
                table.insertBefore(firstChild, x);
                for (var node = tr.firstChild; node; node = next) {
                    next = node.nextSibling;
                    table.insertBefore(node, x);
                }
            }
        } else {
            firstChild = document.createElement("span");
            firstChild.innerHTML = html;
            runScripts(firstChild);
            if (x.parentNode)
                x.parentNode.insertBefore(firstChild, x);
        }
    };

    addNode(x);
    populate(x);
}

function input(x, s, recreate, type, name) {
    if (name) x.name = name;
    if (type) x.type = type;
    x.dead = false;
    x.signal = ss(s);
    x.sources = null;
    x.recreate = recreate(x);
    addNode(x);
    populate(x);

    return x;
}

function inp(s, name) {
    var x = input(document.createElement("input"), s,
                  function(x) { return function(v) { if (x.value != v) x.value = v; }; }, null, name);
    x.value = s.data;
    x.onkeyup = x.oninput = x.onchange = x.onpropertychange = function() { sv(s, x.value) };

    return x;
}

function selectValue(x) {
    if (x.options.length == 0)
        return "";
    else
        return x.options[x.selectedIndex].value;
}

function setSelectValue(x, v) {
  for (var i = 0; i < x.options.length; ++i) {
      if(x.options[i].value == v) {
          x.selectedIndex = i;
          return;
      }
  }
}

function sel(s, content) {
    var dummy = document.createElement("span");
    dummy.innerHTML = "<select>" + content + "</select>";
    var x = input(dummy.firstChild, s, function(x) { return function(v) { if (selectValue(x) != v) setSelectValue(x, v); }; });

    for (var i = 0; i < x.options.length; ++i) {
        if (x.options[i].value == "")
            x.options[i].value = x.options[i].text;
        else
            x.options[i].value = x.options[i].value.substring(1);
    }

    setSelectValue(x, s.data);
    if (selectValue(x) != s.data)
        sv(s, selectValue(x));
    x.onchange = function() { sv(s, selectValue(x)) };

    return x;
}

function chk(s) {
    var x = input(document.createElement("input"), s,
                  function(x) { return function(v) { if (x.checked != v) x.checked = v; }; }, "checkbox");
    x.defaultChecked = x.checked = s.data;
    x.onclick = function() { sv(s, x.checked) };

    return x;
}

function tbx(s) {
    var x = input(document.createElement("textarea"), s,
                  function(x) { return function(v) { if (x.innerHTML != v) x.innerHTML = v; }; });
    x.innerHTML = s.data;
    x.onkeyup = x.oninput = x.onchange = x.onpropertychange = function() { sv(s, x.value) };

    return x;
}

function addOnChange(x, f) {
    var old = x.onchange;
    if (old == null)
        x.onchange = f;
    else
        x.onchange = function() { old(); f(); };
}

function addOnKeyUp(x, f) {
    var old = x.onkeyup;
    if (old == null)
        x.onkeyup = f;
    else
        x.onkeyup = function(x) { old(x); f(x); };
}


// Basic string operations

function eh(x) {
    if (x == null)
        return "NULL";
    else
        return x.split("&").join("&amp;").split("<").join("&lt;").split(">").join("&gt;");
}

function ts(x) { return x.toString() }
function bs(b) { return (b ? "True" : "False") }

function id(x) { return x; }
function sub(s, i) { return s.charAt(i); }
function suf(s, i) { return s.substring(i); }
function slen(s) { return s.length; }
function sidx(s, ch) {
    var r = s.indexOf(ch);
    if (r == -1)
        return null;
    else
        return r;
}
function sspn(s, chs) {
    for (var i = 0; i < s.length; ++i)
        if (chs.indexOf(s.charAt(i)) != -1)
            return i;

    return null;
}
function schr(s, ch) {
    var r = s.indexOf(ch);
    if (r == -1)
        return null;
    else
        return s.substring(r);
}
function ssub(s, start, len) {
    return s.substring(start, start+len);
}

function pi(s) {
    var r = parseInt(s);
    if (r.toString() == s)
        return r;
    else
        er("Can't parse int: " + s);
}

function pfl(s) {
    var r = parseFloat(s);
    if (r.toString() == s)
        return r;
    else
        er("Can't parse float: " + s);
}

function pio(s) {
    var r = parseInt(s);
    if (r.toString() == s)
        return r;
    else
        return null;
}

function pflo(s) {
    var r = parseFloat(s);
    if (r.toString() == s)
        return r;
    else
        return null;
}

function uf(s) {
    if (s.length == 0)
        return "_";
    s = s.replace(new RegExp ("\\.", "g"), ".2E");
    return (s.charAt(0) == '_' ? "_" : "") + encodeURIComponent(s).replace(new RegExp ("%", "g"), ".");
}

function uu(s) {
    if (s.length > 0 && s.charAt(0) == '_') {
        s = s.substring(1);
    } else if (s.length >= 3 && (s.charAt(0) == '%' || s.charAt(0) == '.')
               && s.charAt(1) == '5' && (s.charAt(2) == 'f' || s.charAt(2) == 'F'))
        s = s.substring(3);
    s = s.replace(new RegExp ("\\+", "g"), " ");
    s = s.replace(new RegExp ("\\.", "g"), "%");
    return decodeURIComponent(s);
}

function atr(s) {
    return s.replace(new RegExp ("\"", "g"), "&quot;").replace(new RegExp ("&", "g"), "&amp;")
        }

function ub(b) {
    return b ? "1" : "0";
}

function uul(getToken, getData) {
    var tok = getToken();
    if (tok == "Nil") {
        return null;
    } else if (tok == "Cons") {
        var d = getData();
        var l = uul(getToken, getData);
        return {_1:d, _2:l};
    } else
        whine("Can't unmarshal list (" + tok + ")");
}

function strcmp(str1, str2) {
    return ((str1 == str2) ? 0 : ((str1 > str2) ? 1 : -1));
}


// Remote calls

var client_id = null;
var client_pass = 0;
var url_prefix = "/";
var timeout = 60;

function getXHR(uri)
{
    try {
        return new XMLHttpRequest();
    } catch (e) {
        try {
            return new ActiveXObject("Msxml2.XMLHTTP");
        } catch (e) {
            try {
                return new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {
                whine("Your browser doesn't seem to support AJAX.");
            }
        }
    }
}

var sig = null;

var unloading = false, inFlight = null;

function unload() {
    unloading = true;

    for (; inFlight; inFlight = inFlight.next) {
        inFlight.data.abort();
    }
}

function requestUri(xhr, uri, needsSig) {
    if (unloading)
        return;

    xhr.open("POST", uri, true);
    xhr.setRequestHeader("Content-type", "text/plain");
    xhr.setRequestHeader("Content-length", "0");
    xhr.setRequestHeader("Connection", "close");

    if (client_id != null) {
        xhr.setRequestHeader("UrWeb-Client", client_id.toString());
        xhr.setRequestHeader("UrWeb-Pass", client_pass.toString());
    }

    if (needsSig) {
        if (sig == null)
            whine("Missing cookie signature!");

        xhr.setRequestHeader("UrWeb-Sig", sig);
    }

    inFlight = cons(xhr, inFlight);
    xhr.send(null);
}

function xhrFinished(xhr) {
    xhr.abort();
    inFlight = remove(xhr, inFlight);
}

function unurlify(parse, s) {
    return parse(s);
}

function redirect(s) {
    window.location = s;
}

function rc(prefix, uri, parse, k, needsSig) {
    uri = cat(prefix, uri);
    uri = flattenLocal(uri);
    var xhr = getXHR();

    xhr.onreadystatechange = function() {
        if (xhr.readyState == 4) {
            var isok = false;

            try {
                if (xhr.status == 200)
                    isok = true;
            } catch (e) { }

            if (isok) {
                var lines = xhr.responseText.split("\n");
                if (lines.length != 2)
                    whine("Bad RPC response lines");

                eval(lines[0]);

                try {
                    k(parse(lines[1]));
                } catch (v) {
                    doExn(v);
                }
            } else {
                conn();
            }

            xhrFinished(xhr);
        }
    };

    requestUri(xhr, uri, needsSig);
}

function path_join(s1, s2) {
    if (s1.length > 0 && s1.charAt(s1.length-1) == '/')
        return s1 + s2;
    else
        return s1 + "/" + s2;
}

var channels = [];

function newQueue() {
    return { front : null, back : null };
}
function enqueue(q, v) {
    if (q.front == null) {
        q.front = cons(v, null);
        q.back = q.front;
    } else {
        var node = cons(v, null);
        q.back.next = node;
        q.back = node;
    }
}
function dequeue(q) {
    if (q.front == null)
        return null;
    else {
        var r = q.front.data;
        q.front = q.front.next;
        if (q.front == null)
            q.back = null;
        return r;
    }
}

function newChannel() {
    return { msgs : newQueue(), listeners : newQueue() };
}

function listener() {
    var uri = path_join(url_prefix, ".msgs");
    var xhr = getXHR();
    var tid, orsc, onTimeout;

    var connect = function () {
        xhr.onreadystatechange = orsc;
        tid = window.setTimeout(onTimeout, timeout * 500);
        requestUri(xhr, uri, false);
    }

    orsc = function() {
        if (xhr.readyState == 4) {
            window.clearTimeout(tid);

            var isok = false;

            try {
                if (xhr.status == 200)
                    isok = true;
            } catch (e) { }

            if (isok) {
                var text = xhr.responseText;
                if (text == "")
                    return;
                var lines = text.split("\n");

                if (lines.length < 2) {
                    discon();
                    return;
                }

                var messageReader = function(i) {
                    if (i+1 >= lines.length) {
                        xhrFinished(xhr);
                        connect();
                    }
                    else {
                        var chn = lines[i];
                        var msg = lines[i+1];

                        if (chn == "E") {
                            eval(msg);
                            window.setTimeout(function() { messageReader(i+2); }, 0);
                        } else {
                            if (chn < 0)
                                whine("Out-of-bounds channel in message from remote server");

                            var ch;

                            if (chn >= channels.length || channels[chn] == null) {
                                ch = newChannel();
                                channels[chn] = ch;
                            } else
                                ch = channels[chn];

                            var listener = dequeue(ch.listeners);
                            if (listener == null) {
                                enqueue(ch.msgs, msg);
                            } else {
                                try {
                                    listener(msg);
                                } catch (v) {
                                    doExn(v);
                                }
                            }

                            messageReader(i+2);
                        }
                    }
                }

                messageReader(0);
            }
            else {
                try {
                    if (xhr.status != 0)
                        servErr("Error querying remote server for messages: " + xhr.status);
                } catch (e) { }
            }
        }
    };

    onTimeout = function() {
        xhrFinished(xhr);
        connect();
    };

    connect();
}

function rv(chn, parse, k) {
    if (chn == null)
        return;

    if (chn < 0)
        whine("Out-of-bounds channel receive");

    var ch;

    if (chn >= channels.length || channels[chn] == null) {
        ch = newChannel();
        channels[chn] = ch;
    } else
        ch = channels[chn];

    var msg = dequeue(ch.msgs);
    if (msg == null) {
        enqueue(ch.listeners, function(msg) { k(parse(msg)); });
    } else {
        try {
            k(parse(msg));
        } catch (v) {
            doExn(v);
        }
    }
}

function sl(ms, k) {
    window.setTimeout(function() { k(null); }, ms);
}

function sp(e) {
    execF(e, null);
}


// Key events

var uw_event = null;

function kc() {
    return window.event ? event.keyCode : (uw_event ? uw_event.keyCode : 0);
}


// The Ur interpreter

var urfuncs = [];

function lookup(env, n) {
    while (env != null) {
        if (n == 0)
            return env.data;
        else {
            --n;
            env = env.next;
        }
    }

    whine("Out-of-bounds Ur variable reference");
}

function execP(env, p, v) {
    switch (p.c) {
    case "w":
        return env;
    case "v":
        return cons(v, env);
    case "c":
        if (v == p.v)
            return env;
        else
            return false;
    case "s":
        if (v == null)
            return false;
        else
            return execP(env, p.p, p.n ? v.v : v);
    case "1":
        if (v.n != p.n)
            return false;
        else
            return execP(env, p.p, v.v);
    case "r":
        for (var fs = p.l; fs != null; fs = fs.next) {
            env = execP(env, fs.data.p, v["_" + fs.data.n]);
            if (env == false)
                return false;
        }
        return env;
    default:
        whine("Unknown Ur pattern kind" + p.c);
    }
}

function exec0(env, e) {
    return exec1(env, null, e);
}

function exec1(env, stack, e) {
    var stack, usedK = false;

    var saveEnv = function() {
        if (stack.next != null && stack.next.data.c != "<")
            stack = cons({c: "<", env: env}, stack.next);
        else
            stack = stack.next;
    };

    while (true) {
        switch (e.c) {
        case "c":
            var v = e.v;
            if (stack == null)
                return v;
            var fr = stack.data;

            switch (fr.c) {
            case "s":
                e = {c: "c", v: {v: v}};
                stack = stack.next;
                break;
            case "1":
                e = {c: "c", v: {n: fr.n, v: v}};
                stack = stack.next;
                break;
            case "f":
                fr.args[fr.pos++] = v;
                if (fr.a == null) {
                    var res;
                    stack = stack.next;

                    if (fr.f.apply)
                        res = fr.f.apply(null, fr.args);
                    else if (fr.args.length == 0)
                        res = fr.f();
                    else if (fr.args.length == 1)
                        res = fr.f(fr.args[0]);
                    else if (fr.args.length == 2)
                        res = fr.f(fr.args[0], fr.args[1]);
                    else if (fr.args.length == 3)
                        res = fr.f(fr.args[0], fr.args[1], fr.args[2]);
                    else if (fr.args.length == 4)
                        res = fr.f(fr.args[0], fr.args[1], fr.args[2], fr.args[3]);
                    else if (fr.args.length == 5)
                        res = fr.f(fr.args[0], fr.args[1], fr.args[2], fr.args[3], fr.args[4]);
                    else
                        whine("Native function has " + fr.args.length + " args, but there is no special case for that count.");

                    e = {c: "c", v: res};
                    if (usedK) return null;
                } else {
                    e = fr.a.data;
                    fr.a = fr.a.next;
                }
                break;
            case "a1":
                e = fr.x;
                stack = cons({c: "a2", f: v}, stack.next);
                break;
            case "a2":
                if (fr.f == null)
                    whine("Ur: applying null function");
                else if (fr.f.body) {
                    saveEnv();
                    env = cons(v, fr.f.env);
                    e = fr.f.body;
                } else {
                    e = {c: "c", v: fr.f(v)};
                    stack = stack.next;
                }
                break;
            case "<":
                env = fr.env;
                stack = stack.next;
                break;
            case "r":
                fr.fs["_" + fr.n] = v;
                if (fr.l == null) {
                    e = {c: "c", v: fr.fs};
                    stack = stack.next;
                } else {
                    fr.n = fr.l.data.n;
                    e = fr.l.data.v;
                    fr.l = fr.l.next;
                }
                break;
            case ".":
                e = {c: "c", v: v["_" + fr.f]};
                stack = stack.next;
                break;
            case ";":
                e = fr.e2;
                stack = stack.next;
                break;
            case "=":
                saveEnv();
                env = cons(v, env);
                e = fr.e2;
                break;
            case "m":
                var ps;
                for (ps = fr.p; ps != null; ps = ps.next) {
                    var r = execP(env, ps.data.p, v);
                    if (r != false) {
                        saveEnv();
                        env = r;
                        e = ps.data.b;
                        break;
                    }
                }
                if (ps == null)
                    whine("Match failure in Ur interpretation");
                break;
            default:
                whine("Unknown Ur continuation kind " + fr.c);
            }

            break;
        case "v":
            e = {c: "c", v: lookup(env, e.n)};
            break;
        case "n":
            var idx = e.n;
            e = urfuncs[idx];
            if (e.c == "t")
                e = urfuncs[idx] = eval("(" + e.f + ")");
            break;
        case "s":
            stack = cons({c: "s"}, stack);
            e = e.v;
            break;
        case "1":
            stack = cons({c: "1", n: e.n}, stack);
            e = e.v;
            break;
        case "f":
            if (e.a == null)
                e = {c: "c", v: (eval(e.f))()};
            else {
                var args = [];
                stack = cons({c: "f", f: eval(e.f), args: args, pos: 0, a: e.a.next}, stack);
                if (!e.a.data.c) alert("[2] fr.f = " + e.f + "; 0 = " + e.a.data);
                e = e.a.data;
            }
            break;
        case "l":
            e = {c: "c", v: {env: env, body: e.b}};
            break;
        case "a":
            stack = cons({c: "a1", x: e.x}, stack);
            e = e.f;
            break;
        case "r":
            if (e.l == null)
                whine("Empty Ur record in interpretation");
            var fs = {};
            stack = cons({c: "r", n: e.l.data.n, fs: fs, l: e.l.next}, stack);
            e = e.l.data.v;
            break;
        case ".":
            stack = cons({c: ".", f: e.f}, stack);
            e = e.r;
            break;
        case ";":
            stack = cons({c: ";", e2: e.e2}, stack);
            e = e.e1;
            break;
        case "=":
            stack = cons({c: "=", e2: e.e2}, stack);
            e = e.e1;
            break;
        case "m":
            stack = cons({c: "m", p: e.p}, stack);
            e = e.e;
            break;
        case "e":
            e = {c: "c", v: cs({c: "wc", env: env, body: e.e})};
            break;
        case "wc":
            env = e.env;
            e = e.body;
            break;
        case "K":
            { var savedStack = stack.next, savedEnv = env;
                e = {c: "c", v: function(v) { return exec1(savedEnv, savedStack, {c: "c", v: v}); } };}
            usedK = true;
            break;      
        default:
            whine("Unknown Ur expression kind " + e.c);
        }
    }
}

function execD(e) {
    return exec0(null, e);
}

function exec(e) {
    var r = exec0(null, e);

    if (r != null && r.body != null)
        return function(v) { return exec0(cons(v, r.env), r.body); };
    else
        return r;
}

function execF(f, x) {
    return exec0(cons(x, f.env), f.body);
}


// Wrappers

function confrm(s) {
    return confirm(s) ? true : false;
}


// URL blessing

var urlRules = null;

function checkUrl(s) {
    for (var r = urlRules; r; r = r.next) {
        var ru = r.data;
        if (ru.prefix ? s.indexOf(ru.pattern) == 0 : s == ru.pattern)
            return ru.allow ? s : null;
    }

    return null;
}

function bless(s) {
    u = checkUrl(s);
    if (u == null)
        er("Disallowed URL: " + s);
    return u;
}

// App-specific code