 ring r1=32003,(x,y,z),dp;
ideal i=x,y,z;
ring r2=32003,(a,b),dp;
map f=r1,a,b,a+b;
// maps from r1 to r2,
// x > a
// y > b
// z > a+b
f(i);
==> _[1]=a
==> _[2]=b
==> _[3]=a+b
// operations like f(i[1]) or f(i*i) are not allowed
ideal i=f(i);
// objects in different rings may have the same name
map g = r2,a2,b2;
map phi = g(f);
// composition of map f and g
// maps from r1 to r2,
// x > a2
// y > b2
// z > a2+b2
phi(i);
==> _[1]=a2
==> _[2]=b2
==> _[3]=a2+b2
