anchor.diff:      no ->
           case find(S, Bbs) of
           {ok,V} -> V;
  -        error -> erlang:error({unbound_symb,S})
  +        error -> eval_error({unbound_symb,S})
           end
       end.
   
  
positive.code:                 {yes,B1,Vbs} -> eval_body(B1, add_vbindings(Vbs, Env));
                  no -> apply_match_lambda(Cls, Vals, Env)
              end;
         true -> erlang:error(badarity)
      end;
  apply_match_lambda(_, _, _) -> erlang:error(function_clause).
  
  %% eval_let([PatBindings|Body], Env) -> Value.
  
  
negative.code: eval_case_clauses(V, Cls, Env) ->
      case match_clause(V, Cls, Env) of
          {yes,B,Vbs} -> eval_body(B, add_vbindings(Vbs, Env));
          no -> eval_error({case_clause,V})
      end.
  
  match_clause(V, [[Pat|B0]|Cls], Env) ->
  