Desugared:
Tests.f :: for all a, d, e. [a] -> d -> [e] -> [e]
Tests.f =
    /\ a o86 t97 -> \ x.119 y.120 z.121 ->
	let
	  ds.122 =
	      (++ a) x.119 ((++ a) x.119 ((++ a) x.119 ((++ a) x.119 x.119))) in
	let
	  fail.123 =
	      (error [t97])
		  "\"ds016.hs\", line 17: pattern-matching failed in case\n"S
	in 
	  case ds.122 of {
	    Nil -> Nil! t97
	    (:) a.124 ds.125 ->
		let
		  fail.128 =
		      let bs.126 = ds.125 in
		      let a.127 = a.124 in  (error [t97]) "4"S
		in 
		  case ds.125 of {
		    Nil -> (error [t97]) "2"S
		    (:) b.129 ds.130 ->
			case ds.130 of {
			  (:) c.131 ds.132 ->
			      case ds.132 of {
				Nil ->
				    let a.133 = a.124 in
				    let
				      ds.134 =
					  MkTuple4!
					      o86
					      [t97]
					      o86
					      [t97]
					      y.120
					      z.121
					      y.120
					      z.121
				    in  z.121
				_ ->  fail.128
			      }
			  _ ->  fail.128
			}
		  }
	  }