subst() broken

[Ilya Zakharevich <ilya@math.ohio-state.edu>]

? subst(Mod(1+y,z),y,Mod(x,z))
%78 = 0
? subst(Mod(y,-b*a^2+t^3),y,Mod(x,-b*a^2+t^3))
%75 = 0
? subst(Mod(1+y,-b*a^2+t^3),y,Mod(x,-b*a^2+t^3))
%76 = Mod(Mod(1, -b*a^2 + t^3), -b*a^2 + t^3)

The first two are "just wrong".  The last one is such, and also very
unusable...

Ilya