add comment explaining the bug

Author: sorawee

tests/pyret/regression/tail-recursion-arg-order.arr

@@ -21,11 +21,22 @@ examples:
   oops(4, 5) is working(4, 5)
 end
 
+# When an argument is wrapped in a lambda to delay evaluation, mutation
+# in the tail recursion optimization will be unsound. Consider:
+
 fun oops2(a, b):
   foo = lam(): a end
   if a == 1: b() else: oops2(1, foo) end
 end
 
+# Note that
+#
+# b := lam(): a end
+# a := 1
+#
+# will make the second iteration eveluates to the mutated `a` which is wrong
+# An easy way to fix this is to simply disable TCO in this case
+
 fun working2(a, b):
   foo = lam(): a end
   if a == 1: b() else: working2(1, foo) + 0 end