Add Morphism radd with signature (req ==> req ==> req) as radd_ext1.Add Morphism rmul with signature (req ==> req ==> req) as rmul_ext1.(** Every semi ring can be seen as an almost ring, by taking :
        [-x = x] and [x - y = x + y] *)
 Definition SRopp (x:R) := x.Definition SRsub x y := x + -y.

Lemma SRsub_def : forall x y, x - y == x + -y.