[R6RS] safety.txt clarification

[dyb at cs.indiana.edu]

> Kent proposed a semantics that elevates declarations
> from the level of pragmatics to a device that allows
> syntax-case macros to communicate by attaching 7 bits
> of arbitrary information about the context of every
> identifier.

My intent was to nail down what it means for declarations to have static
scope by proposing a model for how declarations work.

The model happens to allow syntax-case macros access to information about
identifiers only if we include identifier-priority.  This isn't part of
the model proper, but something additional we might want to add if we
agree to the semantics.  In fact, I added identifier-priority as an
afterthought and wish now that I hadn't (yet) since it has caused the
focus to be put on the wrong thing.

Kent