<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def from_callback(cls, cb, nx=None, nparams=None, **kwargs):
 """ Generate a SymbolicSys instance from a callback. Parameters cb : callable Should have the signature ``cb(x, p, backend) -> list of exprs``. nx : int Number of unknowns, when not given it is deduced from ``kwargs['names']``. nparams : int Number of parameters, when not given it is deduced from ``kwargs['param_names']``. \\*\\*kwargs : Keyword arguments passed on to :class:`SymbolicSys`. See also :class:`pyneqsys.NeqSys`. Examples -------- """

        if kwargs.get('x_by_name', False):
            if 'names' not in kwargs:
                raise ValueError("Need ``names`` in kwargs.")
            if nx is None:
                nx = len(kwargs['names'])
            elif nx != len(kwargs['names']):
                raise ValueError("Inconsistency between nx and length of ``names``.")
        if kwargs.get('par_by_name', False):
            if 'param_names' not in kwargs:
                raise ValueError("Need ``param_names`` in kwargs.")
            if nparams is None:
                nparams = len(kwargs['param_names'])
            elif nparams != len(kwargs['param_names']):
                raise ValueError("Inconsistency between ``nparam`` and length of ``param_names``.")

        if nparams is None:
            nparams = 0

        if nx is None:
            raise ValueError("Need ``nx`` of ``names`` together with ``x_by_name==True``.")
        be = Backend(kwargs.pop('backend', None))
        x = be.real_symarray('x', nx)
        p = be.real_symarray('p', nparams)
        _x = dict(zip(kwargs['names'], x)) if kwargs.get('x_by_name', False) else x
        _p = dict(zip(kwargs['param_names'], p)) if kwargs.get('par_by_name', False) else p
        try:
            exprs = cb(_x, _p, be)
        except TypeError:
            exprs = _ensure_3args(cb)(_x, _p, be)
        return cls(x, exprs, p, backend=be, **kwargs)