anchor.diff:    {KnownExtensions, UnknownExtensions} = group_extensions( tl(Parts) ),
   
     ReversedUnknownExtensions = if
  -    UnknownExtensions != [] ->
  +    UnknownExtensions /= [] ->
         UnknownExtensions = lists:reverse(UnknownExtensions);
       true -> []
     end,
  
positive.code: 
  
  compute_extension(SourceFilename, Extension) ->
    if
      compiler_for(Extension) && is_extension_defined(compiler_for(Extension)) ->
        compiler_for(Extension).expected_extension(SourceFilename);
      true -> Extension
    end.
  
  
  is_extension_defined(Module)->
  
negative.code: 
  
  group_extensions([]) ->
    {[], []};
  group_extensions(Extensions) ->
    ReversedExtensions = lists:reverse(Extensions),
    PartitionFunction = fun(Extension)->
      lists:member(Extension, proplists:get_keys(compilers()) )
    end,
    lists:partition(PartitionFunction, ReversedExtensions).
  