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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 _partial_subs(func, func2vars):
 """ Partial-bug proof substitution. Works by making the substitutions on the expression inside the derivative first, and then rebuilding the derivative safely without evaluating it using `_partial_diff`. """

    if isinstance(func, sympy.Derivative):
        new_func = func.expr.xreplace(func2vars)
        new_variables = tuple(var.xreplace(func2vars)
                              for var in func.variables)
        return _partial_diff(new_func, *new_variables)
    else:
        return func.xreplace(func2vars)