[Haskell-cafe] 'par' - why has it the type a -> b -> b ?

What is the reason for implementing parallelism with 'par :: a -> b -> b'? Analogy to 'seq'? I thought parallelism would be introduced most naturally by a function which does two computations in parallel and puts together their results after completion. Say

par2 :: (a -> b -> c) -> (a -> b -> c)

to be used like

par2 (+) expensiveComputationA expensiveComputationB

I assume that par2 can be implemented this way:

par2 f x y = f x (par x y)

?