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

Lemma coherence_sym: (x:A)(y:A)(coherence x y)->(coherence y x).
Proof [x,y:A][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)).