[ #f #Hf #f2 #Hf2 #f1 #Hf #f1a #f1b #Hf1
lapply (coafter_fwd_isid2 … Hf ??) -Hf // #H2f1
elim (sor_inv_isid3 … Hf1) -Hf1 //
- /3 width=5 by coafter_isid_dx, sor_refl, ex3_2_intro/
+ /3 width=5 by coafter_isid_dx, sor_idem, ex3_2_intro/
| #f #_ #IH #f2 #Hf2 #f1 #H1 #f1a #f1b #H2
elim (coafter_inv_xxp … H1) -H1 [1,3: * |*: // ]
[ #g2 #g1 #Hf #Hgf2 #Hgf1