coeffs(P) returns the coefficients of the polynomial P with respect to all the indeterminates of P.

coeffs(P,x) returns the coefficients of the polynomial P with respect to x.

[c, t] = coeffs(P,x) also returns an array of the terms of P.  The terms of P line up such that add(i,i=zip(`*`,a,b)); gives back the original polynomial, P.