<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def linear_exprs(A, x, b=None, rref=False, Matrix=None):
 """ Returns Ax - b Parameters A : matrix_like of numbers Of shape (len(b), len(x)). x : iterable of symbols b : array_like of numbers (default: None) When ``None``, assume zeros of length ``len(x)``. Matrix : class When ``rref == True``: A matrix class which supports slicing, and methods ``__mul__`` and ``rref``. Defaults to ``sympy.Matrix``. rref : bool Calculate the reduced row echelon form of (A | -b). Returns ------- A list of the elements in the resulting column vector. """

    if b is None:
        b = [0]*len(x)
    if rref:
        rA, rb = linear_rref(A, b, Matrix)
        if Matrix is None:
            from sympy import Matrix
        return [lhs - rhs for lhs, rhs in zip(rA * Matrix(len(x), 1, x), rb)]
    else:
        return [sum([x0*x1 for x0, x1 in zip(row, x)]) - v
                for row, v in zip(A, b)]