code: def _nodes_to_values(self):
          
          def is_none(slice):
              return slice[0] == -1 and slice[1] == -1
  
          def get(slice):
              return self.string[slice[0]:slice[1]]
  
          return [(varname, get(slice), slice) for varname, slice in self._nodes_to_regs() if not is_none(slice)]
docs: Returns list of list of (Node, string_value) tuples.