method vterm t = 
    match t.term_node with 
      | Tapp (lf,_ , targs) -> 
          debug "[Calls_collection] app %a" !Ast_printer.d_term t;
          collect_apps lf targs ; SkipChildren
      | Tblock_length ta -> 
          debug "[Calls_collection] block_length %a" !Ast_printer.d_term t;
         collect_apps_builtin [ta] ; SkipChildren
      | _ -> DoChildren