Backquote idioms

[Masinter.pa@Xerox.ARPA]

Many of these issues would be moot if the implementation of backquote
were specified in terms of what the read-macro generated, and the
implementation of the macros that it then expanded to. Implementations
could diverge from the "specified" one only if their divergence was
invisible.

It might mean that some of the current implementations might have to
change, but if the "specified" implementation is faithful in
implementing the book, it couldn't mean too much disruption in existing
code, could it?