>reverse_cons >associative_append @(HFalse ?? Hnotnil)
]
]
-|#ta #tb #tc #Htrue #Hstar #IH #Hout lapply (IH Hout) -IH -Hout #IH whd
+|-ta -tb #ta #tb #tc #Htrue #Hstar #IH #Hout lapply (IH Hout) -IH -Hout #IH whd
#ls #x #xs #end #rs #Hmid_src #Hnotend #Hend #Hnotstart
lapply (refl ? (current ? (nth dst ? ta (niltape ?))))
cases (current ? (nth dst ? ta (niltape ?))) in ⊢ (???%→?);