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Extracting coefficients of multivariate polynomials?

If I enter, say:

P.<x,y,z> = QQ[]
ex = 6*x*y^2-4*x*y*z+2*x^2*z
ex.coefficients()


then Sage returns

[6, -4, 2]


as it should. My problem is that I'm trying to obtain a list of coefficients when the coefficients are themselves expressions:

ex = (1-a^2)*x*y^2+(a-b^2+c)*x*y*z+(b^2-c^2-a)*x^2*z


What I want returned is

[1-a^2, a-b^2+c, b^2-c^2-a]


(the order doesn't matter). If I enter a, b, c as elements of the same polynomial ring as x, y, z then I have the problem of distinguishing the different variables: x, y, z, from a, b, c.

What's the best way to do this?

Thanks, Alasdair