--- a/lib/js/urweb.js	Sat Sep 19 16:18:54 2009 -0400
+++ b/lib/js/urweb.js	Tue Sep 22 09:53:05 2009 -0400
@@ -3,8 +3,18 @@
 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) {
-  return (ls1 ? cons(ls1.data, concat(ls1.next, ls2)) : 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)
@@ -13,13 +23,32 @@
   return false;
 }
 function remove(x, ls) {
-  return (ls ? (ls.data == x ? ls.next : cons(ls.data, remove(x, ls.next))) : null);
+  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) {
-  return (ls1 ? (member(ls1.data, ls2) ? union(ls1.next, ls2) : cons(ls1.data, union(ls1.next, ls2))) : ls2);
+  var acc = ls2;
+
+  for (; ls1; ls1 = ls1.next)
+    if (!member(ls1.data, ls2))
+      acc = cons(ls1.data, acc);
+
+  return acc;
 }
 function length(ls) {
-  return (ls ? 1 + length(ls.next) : 0);
+  var acc = 0;
+
+  for (; ls; ls = ls.next)
+    ++acc;
+
+  return acc;
 }
 
 
@@ -63,6 +92,8 @@
 function doExn(v) {
   if (v == null || v.uw_error == null) {
     var s = (v == null ? "null" : v.toString());
+    if (v != null && v.fileName && v.lineNumber)
+      s += " (" + v.fileName + ":" + v.lineNumber + ")";
     runHandlers("Fail", failHandlers, s);
   }
 }