diff --git a/README.md b/README.md
index a5a5ace..943cdd2 100644
--- a/README.md
+++ b/README.md
@@ -140,11 +140,18 @@ class Foo(EnvExperiment):
 * Type variables are invariant, same as the default in Python. We disallow
   covariant or contravariant. The compiler should mark as error if it encounters
   a type variable used in kernel that is declared covariant or contravariant.
-* Code region protected by a type check, such as `if type(x) == int:`, would
+* A custom function `is_type(x, T)` would be provided to check whether `x` is an
+  instance of `T`, other methods like `type(x) == int` or `isinstance(x, int)`
+  would not compile. The function would be able to check generic types for
+  `list` and `tuple`. When running on the host, user can specify whether to use
+  a debug mode checking (recursively check all elements, which would be slower
+  for large lists) or performance mode which only check the first element of
+  each list. (#15)
+* Code region protected by a type check, such as `if is_type(x, int):`, would
   treat `x` as `int`, similar to how [typescript type guard](https://www.typescripttutorial.net/typescript-tutorial/typescript-type-guards/) works.
   ```python
   def add1(x: Union[int, bool]) -> int:
-    if type(x) == int:
+    if is_type(x, int):
         # x is int
         return x + 1
     else:
@@ -152,7 +159,7 @@ class Foo(EnvExperiment):
         return 2 if x else 1
   ```
 * Generics are instantiated at compile time, all the type checks like
-  `type(x) == int` would be evaluated as constants. Type checks are not allowed
+  `is_type(x, int)` would be evaluated as constants. Type checks are not allowed
   in area outside generics.
 * Type variable cannot occur alone in the result type, i.e. must be bound to the
   input parameters.