Actually limit sequence length.

Fix #1038

lib/ast.js

@@ -633,6 +633,13 @@ var AST_Seq = DEFNODE("Seq", "car cdr", {
             p = p.cdr;
         }
     },
+    len: function() {
+        if (this.cdr instanceof AST_Seq) {
+            return this.cdr.len() + 1;
+        } else {
+            return 2;
+        }
+    },
     _walk: function(visitor) {
         return visitor._visit(this, function(){
             this.car._walk(visitor);