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Notes on implementing deep bindings.

6 Vops:

symbol-value-addr(symbol)
    searches the binding stack for a binding of the supplied symbol and
returns its address or the address of the symbol-value slot.

symbol-value(addr)
set-symbol-value(addr,value)
    gets (or sets) the symbol value when supplied the address returned by
symbol-value-addr.

symbol-boundp(addr)
    predicate testing to see if addr points to the unbound marker.

binding-stack-ref(n)
binding-stack-set(n,value)
    Gets (or sets) the value of the n'th binding on the binding stack.


Transforms:

symbol-value, set, and boundp all have to be specially handled.  If the
symbol is constant, then they can just be converted into ref/set of a
global var.  If the symbol isn't constant, then the symbol-value-addr can't
be done ahead of time.


Refs/Sets of global/special vars:

If the symbol is in the cleanup chain, then use binding-stack-{ref,set}
with the correct offset.  If the symbol isn't on the cleanup chain, then
look up the symbol-value-addr at the start of the function and use
symbol-value/set-symbol-value at the reference.