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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 function_dict(self):
 """ Equivalent to ``self.model_dict``, but with all variables replaced by functions if applicable. Sorted by the evaluation order according to ``self.ordered_symbols``, not alphabetical like ``self.model_dict``! """

        func_dict = OrderedDict()
        for var, func in self.vars_as_functions.items():
            expr = self.model_dict[var].xreplace(self.vars_as_functions)
            func_dict[func] = expr
        return func_dict