fhdl.ir: oops, we defined DomainError twice.

nmigen/fhdl/ir.py

@@ -5,11 +5,7 @@ from .ast import *
 from .cd import *
 
 
-__all__ = ["Fragment", "DomainError"]
-
-
-class DomainError(Exception):
-    pass
+__all__ = ["Fragment"]
 
 
 class Fragment:

nmigen/fhdl/xfrm.py

@@ -3,6 +3,7 @@ from collections import OrderedDict, Iterable
 from ..tools import flatten
 from .ast import *
 from .ast import _StatementList
+from .cd import *
 from .ir import *