<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def find_converting_reactions(model, pair):
 """ 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. """

    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)