PROXY Syntax

[Drew McDermott <Mcdermott at YALE>]

As far as I can see, proxies behave much like handlers.  However, the
former are implemented via an operation PROXY, while the latter are
implemented by the use of (=> ...) clauses in ENTITY definitions.
This asymmetry should be eliminated, either by providing a HANDLER
operation; or by providing (==> proxy) clauses; or both.  I favor
flushing PROXY as an operation, if for no other reason than that
it would never have a default.  Also, the alternative allows constructs
like
   (ENTITY ...
           ...
           (==> PROX1)
           (=> HANDLE)
           (==> PROX2)   )
which would mean, "Send operation to PROX1; if it doesn't like it, try
HANDLE; if it doesn't like it, try PROX2."
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