Note [seqId magic]

'GHC.Prim.seq' is special in several ways.

a) Its fixity is set in GHC.Iface.Load.ghcPrimIface

b) It has quite a bit of desugaring magic.
   See GHC.HsToCore.Utils Note [Desugaring seq] (1) and (2) and (3)

c) There is some special rule handing: Note [User-defined RULES for seq]

Historical note:
    In GHC.Tc.Gen.Expr we used to need a special typing rule for 'seq', to handle calls
    whose second argument had an unboxed type, e.g.  x `seq` 3#

    However, with representation polymorphism we can now give seq the type
    seq :: forall (r :: RuntimeRep) a (b :: TYPE r). a -> b -> b
    which handles this case without special treatment in the typechecker.