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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 permut2expr(self, P):
        ''' Convert a substitution table into an arybo application

        Args:
            P: list of integers. The list must not contain more than 2**nbits elements.

        Returns:
            A tuple containing an :class:`MBAVariable` object with the result
            and the symbolic input variable used in this object. A typical use
            case is to feed these into vectorial_decomp.

        Example:
            >>> mba = MBA(4)
            >>> P = [i^7 for i in range(16)]
            >>> E,X = mba.permut2expr(P)
            >>> E.vectorial_decomp([X])
            App NL = Vec([
            0,
            0,
            0,
            0
            ])
            AffApp matrix = Mat([
            [1, 0, 0, 0]
            [0, 1, 0, 0]
            [0, 0, 1, 0]
            [0, 0, 0, 1]
            ])
            AffApp cst = Vec([
            1,
            1,
            1,
            0
            ])
        '''
        if len(P) > (1<<self.nbits):
            raise ValueError("P must not contain more than %d elements" % (1<<self.nbits))
        X = self.var('X')
        ret = super(MBA, self).permut2expr(P, X.vec)
        return self.from_vec(ret), X