Rationale note about annotation binary syntax

preserves-binary.md

@@ -140,11 +140,26 @@ represent the denoted object, prefixed with `[0x86]`.
 
     «@W V» = [0x85] ++ «W» «V»
 
-Each annotation `W` precedes the `Value` `V` it annotates; both `W` and
-`V` *MAY* themselves be further annotated. Implementations *SHOULD*
-default to omitting annotations from `Repr`s. See [examples in the
+Each annotation `W` precedes the `Value` `V` it
+annotates.[^annotation-syntax-rationale] Both `W` and `V` *MAY*
+themselves be further annotated. Implementations *SHOULD* default to
+omitting annotations from `Repr`s. See [examples in the
 appendix](#annotation-examples).
 
+  [^annotation-syntax-rationale]: **Rationale.** The syntax for
+    annotated values is unlike any other compound syntax defined here.
+    It allows parsers to *statelessly* skip annotations: while `0x85` is
+    the next input byte, skip it, skip a single `Repr`, and then try
+    again; otherwise, parse the input stream as usual.
+
+    By contrast, one might imagine using a parenthesized syntax like the syntax of
+    compound data. In that case, parsers would have to remember
+    information about nesting of annotations. This would not necessarily
+    be an issue for recursive ("DOM"-like) parsers, but streaming
+    ("SAX"-like) parsers would have to either take on complexity
+    internally or force it on their clients, even when those clients did
+    not care about annotations.
+
 ## Security Considerations
 
 **Annotations.** In modes where a `Value` is being read while