[ILibraryView / Pattern] flatten() should raise PatternError if asked to preserve ports from a repeated ref

masque/library.py

@@ -308,6 +308,11 @@ class ILibraryView(Mapping[str, 'Pattern'], metaclass=ABCMeta):
                     continue
 
                 for ref in pat.refs[target]:
+                    if flatten_ports and ref.repetition is not None and target_pat.ports:
+                        raise PatternError(
+                            f'Cannot flatten ports from repeated ref to {target!r}; '
+                            'flatten with flatten_ports=False or expand/rename the ports manually first.'
+                            )
                     p = ref.as_pattern(pattern=target_pat)
                     if not flatten_ports:
                         p.ports.clear()