+Note [NoSpecConstr]
+~~~~~~~~~~~~~~~~~~~
+The ignoreAltCon stuff allows you to say
+ {-# ANN type T NoSpecConstr #-}
+to mean "don't specialise on arguments of this type. It was added
+before we had ForceSpecConstr. Lacking ForceSpecConstr we specialised
+regardless of size; and then we needed a way to turn that *off*. Now
+that we have ForceSpecConstr, this NoSpecConstr is probably redundant.
+(Used only for PArray.)
+