<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_all_transport_reactions(model, boundaries, allow_duplicates=False):
 """Add all transport reactions to database and to model. Add transport reactions for all boundaries. Boundaries are defined by pairs (2-tuples) of compartment IDs. Transport reactions are added for all compounds in the model, not just for compounds in the two boundary compartments. Args: model: :class:`psamm.metabolicmodel.MetabolicModel`. boundaries: Set of compartment boundary pairs. Returns: Set of IDs of reactions that were added. """


    all_reactions = {}
    if not allow_duplicates:
        # TODO: Avoid adding reactions that already exist in the database.
        # This should be integrated in the database.
        for rxnid in model.database.reactions:
            rx = model.database.get_reaction(rxnid)
            all_reactions[rx] = rxnid

    boundary_pairs = set()
    for source, dest in boundaries:
        if source != dest:
            boundary_pairs.add(tuple(sorted((source, dest))))

    added = set()
    added_pairs = set()
    initial_compounds = set(model.compounds)
    reactions = set(model.database.reactions)
    for compound in initial_compounds:
        for c1, c2 in boundary_pairs:
            compound1 = compound.in_compartment(c1)
            compound2 = compound.in_compartment(c2)
            pair = compound1, compound2
            if pair in added_pairs:
                continue

            rxnid_tp = create_transport_id(reactions, compound1, compound2)

            reaction_tp = Reaction(Direction.Both, {
                compound1: -1,
                compound2: 1
            })
            if reaction_tp not in all_reactions:
                model.database.set_reaction(rxnid_tp, reaction_tp)
                reactions.add(rxnid_tp)
            else:
                rxnid_tp = all_reactions[reaction_tp]

            if not model.has_reaction(rxnid_tp):
                added.add(rxnid_tp)
            model.add_reaction(rxnid_tp)
            added_pairs.add(pair)

    return added