code: def _extend_conversions(
      mtype, anchor_mtype, convert_dict, mtype_universe=None
  ):
      """Obtain all conversions from and to mtype via conversion to anchor_mtype.
  
      Mutates convert_dict by adding all conversions from and to mtype.
  
      Assumes:
      convert_dict contains
      * conversion from `mtype` to `anchor_mtype`
      * conversion from `anchor_mtype` to `mtype`
      * conversions from `anchor_mtype` to all mtypes in `mtype_universe`
      * conversions from all mtypes in `mtype_universe` to `anchor_mtype`
  
      Guarantees:
      convert_dict contains
      * conversions from `mtype` to all mtypes in mtype_universe
      * conversions from all mtypes in mtype_universe to `mtype`
  
      conversions <mtype to mtype2> not in convert_dict at start are filled in as
      _concat(<from mtype to anchor_type>, <from anchor_type to mtype2>)
      conversions <mtype2 to mtype> not in convert_dict at start are filled in as
      _concat(<from mtype2 to anchor_type>, <from anchor_type to mtype>)
  
      Parameters
      ----------
      mtype : mtype string in convert_dict
      anchor_mtype : mtype string in convert_dict
      convert_dict : conversion dictionary with entries of converter signature
          see docstring of datatypes._convert
      mtype_universe : iterable of mtype strings in convert_dict, coercible to list or set
  
      Returns
      -------
      reference to convert_dict
      CAVEAT: convert_dict passed to this function gets mutated, this is a reference
      """
      keys = convert_dict.keys()
      scitype = list(keys)[0][2]
  
      if mtype_universe is None:
          mtype_universe = {x[1] for x in list(keys)}
          mtype_universe = mtype_universe.union(
              [x[0] for x in list(keys)]
          )
  
      for tp in set(mtype_universe).difference([mtype, anchor_mtype]):
          if (anchor_mtype, tp, scitype) in convert_dict.keys():
              if (mtype, tp, scitype) not in convert_dict.keys():
                  convert_dict[(mtype, tp, scitype)] = _concat(
                      convert_dict[(mtype, anchor_mtype, scitype)],
                      convert_dict[(anchor_mtype, tp, scitype)],
                  )
          if (tp, anchor_mtype, scitype) in convert_dict.keys():
              if (tp, mtype, scitype) not in convert_dict.keys():
                  convert_dict[(tp, mtype, scitype)] = _concat(
                      convert_dict[(tp, anchor_mtype, scitype)],
                      convert_dict[(anchor_mtype, mtype, scitype)],
                  )
  
      return convert_dict
  
label_desc: Use image node_id 2 for calling a global function with example usage: _extend_conversions(mtype, anchor_mtype, convert_dict, mtype_universe) and returns: convert_dict