> Maybe provide a small example as a tested for experiment.
On a small example one can proceed as follows :
minpoly_for_h=h^6 - 3*h^4 + 3*h^2 - 3
minpoly_for_g=g^3-2
relator=(15*h^4 - 18*h^2 - 15)*g^2 + (6*h^5 - 12*h^3 + 18*h^2 - 30*h - 30)*g + (36*h^3 - 18*h^2 - 48*h - 6)
polrem(pol1,pol2,var)=divrem(pol1,pol2,var)[2]
relator_lead_inverse=lift(Mod(1/pollead(relator,g),minpoly_for_h))
relator2=polrem(relator_lead_inverse*relator,minpoly_for_h,h)
relator3=polrem(minpoly_for_g,relator2,g)
relator4=polrem(relator3,minpoly_for_h,h)
g_in_terms_of_h=lift(Mod(-polcoeff(relator4,0,g)/polcoeff(relator4,1,g),minpoly_for_h))
Cheers,
Ewan