code: def find_converting_reactions(model, pair):
      
      first = set(find_met_in_model(model, pair[0]))
      second = set(find_met_in_model(model, pair[1]))
      hits = list()
      for rxn in model.reactions:
          # FIXME: Use `set.issubset` much more idiomatic.
          if len(first & set(rxn.reactants)) > 0 and len(
                  second & set(rxn.products)) > 0:
              hits.append(rxn)
          elif len(first & set(rxn.products)) > 0 and len(
                  second & set(rxn.reactants)) > 0:
              hits.append(rxn)
      return frozenset(hits)
docs: Find all reactions which convert a given metabolite pair.
  
      Parameters
      ----------
      model : cobra.Model
          The metabolic model under investigation.
      pair: tuple or list
          A pair of metabolite identifiers without compartment suffix.
  
      Returns
      -------
      frozenset
          The set of reactions that have one of the pair on their left-hand
          side and the other on the right-hand side.