anchor.diff:  
   substitute({tuple, Types}, Substitutions) ->
       {tuple, substitute(Types, Substitutions)};
  +
  +substitute({builtin_type, Type}, _Substitutions) ->
  +    {builtin_type, Type};
   
   substitute({union, _Line, Types}, Substitutions) ->
       {union, substitute(Types, Substitutions)};
  
positive.code: %
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  
  -spec substitutions_from_list(list({var, integer(), atom()}), list(term())) 
                               -> gb_trees:tree({var, atom()}, term()).
  substitutions_from_list(VarList, TypeList) ->
      Tree = gb_trees:empty(),
  
negative.code: %% @author Thorsten Schuett <schuett@zib.de>
  %% @doc    variable substitutions for tester
  %% @end
  -module(tester_variable_substitutions).
  -author('schuett@zib.de').
  
  -include("unittest.hrl").
  -include("tester.hrl").
  