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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 constraint_to_si(expr):
 """ Convert a constraint to SI if possible. :param expr: :return: """


    satisfiable = True
    replace_list = [ ]

    satisfiable, replace_list = backends.vsa.constraint_to_si(expr)

    # Make sure the replace_list are all ast.bvs
    for i in xrange(len(replace_list)):
        ori, new = replace_list[i]
        if not isinstance(new, Base):
            new = BVS(new.name, new._bits, min=new._lower_bound, max=new._upper_bound, stride=new._stride, explicit_name=True)
            replace_list[i] = (ori, new)

    return satisfiable, replace_list