Theorem coherence_intro :  (x:A)(y:A)(z:A)(Rstar x z)->(Rstar y z)->(coherence x y).
Proof [x,y:A]
[z:A]
[H:(Rstar x z)]
[H0:(Rstar y z)]
(unf_coherence [P:A->A->Prop](P x y)
[P:Prop][H1:(z0:A)(Rstar x z0)->(Rstar y z0)->P](H1 z H H0)).