anchor.diff:  
           let
   
  -          trace = x: builtins.trace x x;
  -
             deps = import (pkgs.runCommand "lhs2tex-includes"
  -            { src = trace key; }
  +            { src = key; }
               "${pkgs.stdenv.bash}/bin/bash ${./find-lhs2tex-includes.sh}");
   
  -        in pkgs.lib.concatMap (x : if builtins.pathExists x then [{key = x;}]
  -                                   else builtins.trace ("not found: ${toString x}") [])
  -                              (map (x : trace "${dirOf key}/${x}") deps);
  +        in pkgs.lib.concatMap (x : if builtins.pathExists x then [{key = x;}] else [])
  +                              (map (x : "${dirOf key}/${x}") deps);
       };
   
     dot2pdf =
  
positive.code:               fn = pkgs.lib.findFirst (fn: builtins.pathExists fn) null
                  (map (ext: "${dirOf key}/${dep.name}${ext}") exts);
              in if fn != null then [{key = fn;}] ++ xs
                 else builtins.trace "not found: ${dep.name}" xs;
  
          in pkgs.lib.fold foundDeps [] deps;
      };
  
negative.code: 
          let
  
            # `find-includes.pl' returns the dependencies of the current
            # source file (`key') as a list, e.g. [{type = "tex"; name =
            # "introduction.tex";} {type = "img"; name = "example"}].
            # The type denotes the kind of dependency, which determines
            # what extensions we use to look for it.
            deps = import (pkgs.runCommand "latex-includes"
              { src = key; }
              "${pkgs.perl}/bin/perl ${./find-includes.pl}");
  
            # Look for the dependencies of `key', trying various
  