/**/  use R ::= QQ[x,y,z];
/**/  S ::= QQ[x[1..3]];
/**/  phi := PolyAlgebraHom(R, S, indets(S));
/**/  phi([x^2-y, z-2]);         -- was  apply(phi, [x^2-y, z-2])  up to 5.3.3
[x[1]^2 -x[2], x[3] -2]
/**/  phi(mat([[x,1], [y,z]]));  -- was  apply(phi, mat(...))   up to 5.3.3