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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def find_exchange_rxns(model):
 u""" Return a list of exchange reactions. Parameters model : cobra.Model The metabolic model under investigation. Notes ----- [1] defines exchange reactions as: -- reactions that 'define the extracellular environment' -- 'unbalanced, extra-organism reactions that represent the supply to or removal of metabolites from the extra-organism "space"' -- reactions with a formula such as: 'met_e -> ' or ' -> met_e' or 'met_e <=> ' Exchange reactions differ from demand reactions in that the metabolites are removed from or added to the extracellular environment only. With this the uptake or secretion of a metabolite is modeled, respectively. References .. [1] Thiele, I., & Palsson, B. Ø. (2010, January). A protocol for generating a high-quality genome-scale metabolic reconstruction. Nature protocols. Nature Publishing Group. http://doi.org/10.1038/nprot.2009.203 """

    try:
        extracellular = find_compartment_id_in_model(model, 'e')
    except KeyError:
        extracellular = None
    return find_boundary_types(model, 'exchange', extracellular)