def a(x=3, b, c) should not be accepted

But accepted in gpython
it should be raised Syntax error.

## cpython
``` python
>>> def a(x=3, b, c):
... pass
...
File "", line 1
SyntaxError: non-default argument follows default argument
````

## gpython
```python
>>> def a(x=3, b, c):
... pass
...
>>> a()
Traceback (most recent call last):
File "", line 1, in
FIXME line of source goes here
TypeError: "a() missing 2 required positional arguments: 'x' and 'b'"
```