[Json] Response to Statement from W3C TAG

[Joseph Boyle <planetwork@josephboyle.net>]

https://mail.mozilla.org/pipermail/es-discuss/2013-December/035126.html

This provides the term "static semantic rule" to describe non-ABNF rules such as:
- the subject and object of a contextual statement must yield a valid XDI address when concatenated by removing the // between them
- a statement whose object is parentheses enclosing a syntactically valid XDI statement, is semantically equivalent to a rearranged statement using inner roots

The first does restrict the set of syntactically valid XDI statements, even thought it is called a semantic rule. The second does not reduce the set of all valid XDI statements, or could be viewed as increasing it from a more basic smaller set not allowing triples in parentheses as objects. 

The mail gives ignoring insignificant whitespace as another example of a static semantic rule, which could alternatively be implemented using syntax diagrams ("racetrack diagrams", which are equivalent to BNF). In the XDI case we have never attempted to add it to the ABNF, but have always had it as a static semantic rule external to the ABNF.

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