Add note about endianness of repr_J.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -4264,6 +4264,12 @@ Define $\reprJ \typecolon \GroupJ \rightarrow \bitseq{\ellJ}$ such
 that $\reprJ(u, \varv) = \ItoLEBSP{256}(\varv + 2^{255} \smult \tilde{u})$, where
 $\tilde{u} = u \bmod 2$.
 
+\todo{Representing this as a bit string is problematic because we normally encode
+most-significant-bit first within a byte, so that would result in the wrong
+(i.e. non-standard) encoding as a byte sequence. It's a tricky specification
+problem that we get away with elsewhere in the spec mostly by luck. Maybe keep
+the representation as an integer?}
+
 Let $\abstJ \typecolon \bitseq{\ellJ} \rightarrow \GroupJ \union \setof{\bot}$
 be the left inverse of $\reprJ$ such that if $S$ is not in the range of
 $\reprJ$, then $\abstJ(S) = \bot$.