Use R ::= QQ[x,y,z,t];
F := x +y +z +t^2;
Subst(F, x, -2);
t^2 + y + z - 2
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Subst(F, x, z/y);
(yt^2 + y^2 + yz + z)/y
-------------------------------
Subst(F, [[x,x^2], [y,y^3], [z,t^5]]);
t^5 + y^3 + x^2 + t^2
-------------------------------
Eval(F, [x^2,y^3,t^5]); -- the same thing as above
t^5 + y^3 + x^2 + t^2
-------------------------------
MySubst := [[y,1], [t,3z-x]];
Subst(xyzt, MySubst); -- substitute into the function xyzt
-x^2z + 3xz^2
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