I was a bit surprised that ratsubst didn't crunch %i80 to zero:
(%i80) ratsubst(6*z^2+37*z-170,z^3,-6*z^4+33*z^3+240*z^2-909*z-510);
(%o80) -36*z^3+216*z^2+1332*z-6120
(%i82) load ("lrats")$
(%i83) fullratsubst(6*z^2+37*z-170,z^3,-6*z^4+33*z^3+240*z^2-909*z-510);
(%o83) 0
Maybe the user documentation for ratsubst should mention that ratsubst might
need to be applied more than once?
--Barton