The restriction of the residue class map to R is the map
j : R -> R[X]/(d),
a -> a + (d).
R[X] is a polynomial
of positive degree.
The injectivity tells us that within R[X]/(d) we find
the `copy' j(R) of R, where the term copy refers
not only to the bijective correspondence between the sets R
and j(R), but also refers to the fact that
j respects the operations addition and product.