def common_start(chain_iters, just_replace):
    """ Returns modified `chain_iters` and `just_replace` such that the
 calculations will start at the first value of each variable in chain_iter
    """
    chain_iters_out = chain_iters.copy()
    replacers = {}
    if len(chain_iters_out) > 0:
        for key, elem in chain_iters_out.items():
            replacers[key] = elem.pop(0)
        else:
            elem.insert(0, replacers[key])
    just_replace = just_replace.copy()
    just_replace.update(replacers)
    return chain_iters_out, just_replace
