anchor.diff:  
   check_undefined_function_calls(_Config) ->
       {ok, UndefinedCalls0} = xref:analyze(xref, undefined_function_calls),
  -
       UndefinedCalls = [{find_mfa_source(Caller), format_fa(Caller), format_mfa(Target)} ||
                            {Caller, Target} <- UndefinedCalls0],
       [?CONSOLE("~s:~w: Warning ~s calls undefined function ~s\n",
  
positive.code:                             {verbose, rebar_config:is_verbose()}]),
      {ok, _} = xref:add_directory(xref, "ebin"),
  
      %% Get list of xref checks we want to run
      XrefChecks = rebar_config:get(Config, xref_checks, [exports_not_used,
                                                          undefined_function_calls]),
  
negative.code:     Source = abstract_code_source_file(Code),
      Line = abstract_code_function_line(Code, F, A),
      {Source, Line}.
  
  