Add simple curve cycle trait

ec/src/lib.rs

@@ -322,3 +322,13 @@ where
  Fq = <Self::E1 as PairingEngine>::Fr,
  >;
 }
+
+/// A cycle of elliptic curves, unopinionated
+pub trait SimpleCycleEngine: Sized + 'static + Copy + Debug + Sync + Send
+where
+ <Self::E1 as AffineCurve>::Projective: MulAssign<<Self::E2 as AffineCurve>::BaseField>,
+ <Self::E2 as AffineCurve>::Projective: MulAssign<<Self::E1 as AffineCurve>::BaseField>,
+{
+ type E1: AffineCurve;
+ type E2: AffineCurve;
+}