anchor.diff:  
      module  mkReducePipes#(CombinePipe#(Tuple2#(a,a), a) combinepipe,
   			  Vector#(n, PipeOut#(a)) inpipes)
  -			 (PipeOut#(a) ifc);
  +			 (PipeOut#(a));
         Vector#(TDiv#(n,2),PipeOut#(a)) pipes0 = takeAt(0, inpipes);
         Vector#(TSub#(n,TDiv#(n,2)),PipeOut#(a)) pipes1 = takeAt(valueOf(TDiv#(n,2)), inpipes);
   
  
positive.code: typeclass ReducePipe#( numeric type n, type a);
     module  mkReducePipe#(CombinePipe#(Tuple2#(a,a), a) combinepipe,
  			 PipeOut#(Vector#(n,a)) inpipe)
  			 (PipeOut#(a) ifc);
     module  mkReducePipes#(CombinePipe#(Tuple2#(a,a), a) combinepipe,
  			  Vector#(n,PipeOut#(a)) inpipe)
  			  (PipeOut#(a) ifc);
  endtypeclass
  instance ReducePipe#(1, a);
     module  mkReducePipe#(CombinePipe#(Tuple2#(a,a), a) combinepipe,
  				 PipeOut#(Vector#(1,a)) inpipe)
  				 (PipeOut#(a) ifc);
        let pipe = mapPipe(head, inpipe);
        return pipe;
     endmodule
     module  mkReducePipes#(CombinePipe#(Tuple2#(a,a), a) combinepipe,
  				  Vector#(1,PipeOut#(a)) inpipes)
  				  (PipeOut#(a) ifc);
        return inpipes[0];
     endmodule
  endinstance
  
negative.code:    provisos(Bits#(a,a__));
     module  mkReducePipe#(CombinePipe#(Tuple2#(a,a), a) combinepipe,
  			 PipeOut#(Vector#(2,a)) inpipe)
  			(PipeOut#(a));
        function a foo(Vector#(2,a) invec); 
  	 return combinepipe(tuple2(invec[0], invec[1])); 
        endfunction
  