disallow default parameter

README.md

@@ -51,17 +51,9 @@ class Foo(EnvExperiment):
 * No implicit coercion, require implicit cast. Integers are int32 by default,
   floating point numbers are double by default.
 * RPCs: optional parameter type signature, require return type signature.
-* Value of the optional parameter would be created by the caller, and
-  independent between different invokations. The following code would always
-  output 2 `1`, instead of `1` and `2` for actual Python:
-  ```py
-  def test(a = [1]):
-      print(a)
-      a[0] += 1
-
-  test()
-  test()
-  ```
+* Function default parameters are not allowed, as changes to the default value
+  would not be kept across kernel calls, and that is a potential source of
+  confusion.
 
 ## Generics
 We use [type variable](https://docs.python.org/3/library/typing.html#typing.TypeVar) for denoting generics.