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+# SimdShape
+
+A logical extension of the nmigen `ast.Shape` concept, `SimdShape`
+provides sufficient context to both define overrides for individual lengths
+on a per-mask basis as well as sufficient information to "upcast"
+back to a SimdSignal, in exactly the same way that c++ virtual base
+class upcasting works when RTTI (Run Time Type Information) works.