code: def _register_instruction_in_defs(self, instruction):
          
          type_ = instruction.type
          color_ = instruction.color
          instruction_to_svg_dict = \
              self._instruction_to_svg.instruction_to_svg_dict
          instruction_id = "{}:{}".format(type_, color_)
          defs_id = instruction_id + ":defs"
          if instruction_id not in self._instruction_type_color_to_symbol:
              svg_dict = instruction_to_svg_dict(instruction)
              self._compute_scale(instruction_id, svg_dict)
              symbol = self._make_definition(svg_dict, instruction_id)
              self._instruction_type_color_to_symbol[defs_id] = \
                  symbol[DEFINITION_HOLDER].pop("defs", {})
              self._instruction_type_color_to_symbol[instruction_id] = symbol
          return instruction_id
docs: Create a definition for the instruction.
  
          :return: the id of a symbol in the defs for the specified
            :paramref:`instruction`
          :rtype: str
  
          If no symbol yet exists in the defs for the :paramref:`instruction` a
          symbol is created and saved using :meth:`_make_symbol`.