[MLton] redundant case rules warning

[Lukasz S Ziarek]

Could someone enlighten me to why the following case has redundant rules.
I am casing on the primitive name in a prim app in the SSA il.

case Prim.name prim
  of Ref_ref => ...
   | Ref_dref => ...
   | _ => ...


Warning: ssa/hoist.fun 274.43:
   Case has redundant rules.
   rules: Ref_dref => setAlias (n, addAlias (  ...  lias))))), alias),
store)
          _ => setAlias (n, alias, store)
   in: case Prim.name prim of (Ref_ref) =>  ...  tAlias (n, alias, store))


thanks,
Luke