End ALMOST_RING.
Variable Rth : ring_theory 0 1 radd rmul rsub ropp req.
Let ARth := Rth_ARth Rsth Reqe Rth.
Add Morphism rsub with signature (req ==> req ==> req) as rsub_ext4.

Lemma Nth : semi_ring_theory 0%N 1%N N.add N.mul (@eq N).
Definition Nsub := SRsub N.add.