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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 is_constrained_reaction(model, rxn):
 """Return whether a reaction has fixed constraints."""

    lower_bound, upper_bound = helpers.find_bounds(model)
    if rxn.reversibility:
        return rxn.lower_bound > lower_bound or rxn.upper_bound < upper_bound
    else:
        return rxn.lower_bound > 0 or rxn.upper_bound < upper_bound