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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 from_callback(cls, cb, transf_cbs, nx, nparams=0, pre_adj=None, **kwargs):
 """ Generate a TransformedSys instance from a callback Parameters cb : callable Should have the signature ``cb(x, p, backend) -> list of exprs``. The callback ``cb`` should return *untransformed* expressions. transf_cbs : pair or iterable of pairs of callables Callables for forward- and backward-transformations. Each callable should take a single parameter (expression) and return a single expression. nx : int Number of unkowns. nparams : int Number of parameters. pre_adj : callable, optional To tweak expression prior to transformation. Takes a sinlge argument (expression) and return a single argument rewritten expression. \\*\\*kwargs : Keyword arguments passed on to :class:`TransformedSys`. See also :class:`SymbolicSys` and :class:`pyneqsys.NeqSys`. Examples -------- """

        be = Backend(kwargs.pop('backend', None))
        x = be.real_symarray('x', nx)
        p = be.real_symarray('p', nparams)
        try:
            transf = [(transf_cbs[idx][0](xi),
                       transf_cbs[idx][1](xi))
                      for idx, xi in enumerate(x)]
        except TypeError:
            transf = zip(_map2(transf_cbs[0], x), _map2(transf_cbs[1], x))
        try:
            exprs = cb(x, p, be)
        except TypeError:
            exprs = _ensure_3args(cb)(x, p, be)
        return cls(x, _map2l(pre_adj, exprs), transf, p, backend=be, **kwargs)