add useful debug statement for when domains already exist

diff --git a/nmigen/hdl/ir.py b/nmigen/hdl/ir.py
index 3c1ec6045d945fc6461d90eda08e5f48dfbb976e..5a167f91428adc4e23da75505a1cbece4f1e88fc 100644 (file)
--- a/nmigen/hdl/ir.py
+++ b/nmigen/hdl/ir.py
@@ -112,7 +112,8 @@ class Fragment:
     def add_domains(self, *domains):
         for domain in flatten(domains):
             assert isinstance(domain, ClockDomain)
-            assert domain.name not in self.domains
+            assert domain.name not in self.domains, \
+            "domain.name %s already in %s" % (domain.name, repr(self.domains))
             self.domains[domain.name] = domain
 
     def iter_domains(self):