Coefficients in a multivariate polynomial?

Suppose I want to find the coefficient of x^2y in the expansion of

(x-2*y+3*z-4)^5

I can do:

(%i1) ex:expand((x-2*y+3*z-4)^5);
(%i2) coeff(coeff(coeff(ex,x,2),y,1),z,0);

But surely there must be a simpler way than calling three nested coeff's?

What I have is an expression which may be considered as a multivariate
polynomial, the coefficients being themselves expressions; something like

(1-a^2)xy-(a-b)x^2y+(a-b^2+c)xy^2+...

I need to be able to extract the coefficients: 1-a^2, a-b, a-b^2+c from
this expression.  Is there an easy way to do this in Maxima?

Thanks,
Alasdair

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