Desugared:
{- plain CoRec -}
MapPairs.mappairs :: for all d, e, f. (d -> e -> f) -> [d] -> [e] -> [f]
MapPairs.mappairs =
    /\ t109 t112 t105 -> \ f.146 ds.147 ys.148 ->
	let
	  fail.149 =
	      (error [t105])
		  "\"ds005.hs\", line 10: pattern-matching failure [function binding]\n"S
	in 
	  case ds.147 of {
	    Nil -> Nil! t105
	    (:) x.150 xs.151 ->
		case ys.148 of {
		  Nil -> let f.152 = f.146 in  Nil! t105
		  (:) y.153 ys.154 ->
		      let xs.155 = xs.151 in
		      let x.156 = x.150 in
		      let f.157 = f.146
		      in 
			(: t105)
			    (f.157 x.156 y.153)
			    ((((MapPairs.mappairs t109) t112) t105)
				 f.157 xs.155 ys.154)
		}
	  }
{- end plain CoRec -}
{- plain CoRec -}
MapPairs.mappairs' :: for all d, e, f. (d -> e -> f) -> [d] -> [e] -> [f]
MapPairs.mappairs' =
    /\ t133 t136 t129 -> \ f.162 ds.163 ys.164 ->
	let
	  fail.165 =
	      (error [t129])
		  "\"ds005.hs\", line 15: pattern-matching failure [function binding]\n"S in
	let
	  fail.174 =
	      let
		fail.171 =
		    case ds.163 of {
		      (:) x.166 xs.167 ->
			  case ys.164 of {
			    (:) y.168 ys.169 ->
				let f.170 = f.162
				in 
				  (: t129)
				      (f.170 x.166 y.168)
				      ((((MapPairs.mappairs' t133) t136) t129)
					   f.170 xs.167 ys.169)
			    _ ->  fail.165
			  }
		      _ ->  fail.165
		    }
	      in 
		case ys.164 of {
		  Nil -> let x.172 = ds.163 in let f.173 = f.162 in  Nil! t129
		  _ ->  fail.171
		}
	in 
	  case ds.163 of {
	    Nil -> Nil! t129
	    _ ->  fail.174
	  }
{- end plain CoRec -}