Implementing Haskell's Type-Level Quicksort in D

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Coming back to this after a few days. I got a bit farther, but 
I'm running into trouble with the template args pattern matching. 
I'd like to turn this code:

  list1 :: Cons Three (Cons Two (Cons Four (Cons One Nil)))
  list1 = undefined

  numlHead :: Cons a b -> a
  numlHead = const undefined

  numlTail :: Cons a b -> b
  numlTail = const undefined'


Into this D code:

alias list1 = Cons!(Three, Cons!(Two, Cons!(Four, Cons!(One, 
Nil))));

alias numlHead(L: Cons!(a, b), a, b) = a;

alias numlTail(L: Cons!(a, b), a, b) = b;


But the compiler is complaining loudly about a mismatch:

/d43/f234.d(39): Error: template instance numlHead!(list1) does 
not match template declaration numlHead(L : Cons!(a, b), a, b)