Lemma coherence_sym: (x:A)(y:A)(coherence x y)->(coherence y x).
Proof [x,y:A]
(unf_coherence [P:A->A->Prop](P x y)->(P y x)
[H:(P:Prop)((z:A)(Rstar x z)->(Rstar y z)->P)->P]
[P:Prop]
[H0:(z:A)(Rstar y z)->(Rstar x z)->P]
(H P [z:A][H1:(Rstar x z)][H2:(Rstar y z)](H0 z H2 H1))).