add lengthPrf0 for contexts

src/Quox/Context.idr

@@ -183,6 +183,12 @@ lengthPrf [<]        = (0 ** Refl)
 lengthPrf (tel :< _) =
   let len = lengthPrf tel in (S len.fst ** cong S len.snd)
 
+export
+lengthPrf0 : Context _ to -> (len : Nat ** len = to)
+lengthPrf0 ctx =
+  let (len ** prf) = lengthPrf ctx in
+  (len ** rewrite sym $ plusZeroRightNeutral len in prf)
+
 public export %inline
 length : Telescope {} -> Nat
 length = fst . lengthPrf