Description of closconv.scm

Purpose:
--------
CLOSCONV is the closure converter.  It rewrites instances of lambda
expressions with free variables into calls to the pseudo-primitives
%make-closure (and %make-trivial-closure).  References to free
variables of closures are rewritten as calls using the
pseudo-primitive %closure-ref (and %vector-index).  
In a LETREC for closed-over env every ordinary-ref of a non-closed-over 
env, is changed into a CALL to %make-trivial-closure.
Every free-ref (closed over) is replaced with %closure-ref and 
%vector-index. Also if the free ref variable is in the closed-over-names list
produced by a call to the same procedure that rewrites lambda 
then %heap-closure-set is called. 

CLOSCONV, like LAMLIFT, is called twice, before and after CPS conversion. 
Before CPS conversion, it uses %make-heap-closure and %heap-closure-ref
for %make-closure and %closure-ref.  After CPS conversion, it uses 
%make-stack-closure and %stack-closure-ref instead.

  After LAMLIFT and CLOSCONV, the only free variables in lambda
expressions are those bound to static bindings (always available, e.g.
from the program counter).

Operators Introduced:
---------------------
%make-closure replaces free variables in LAMBDA and LETREC
%make-heap-closure
%make-trivial-closure constructs an externally callable procedure object 
(all free variables) are accessible through the variable caching mechanism
.A LOOKUP is permitted only in a LETREC at the top level of a
program.  It is used to export one of the mutually recursive
procedures introduced by the LETREC to the external environment.
%make-stack-closure This appears *only* as the continuation of some 
KMP-Scheme CALL. If a lambda-expression is supplied, it pushes the 
values on the stack (creating a stack closure of the format specified) and
loads the return address specified by the lambda-expression
into the return address location (register or stack
location).  If no lambda expression is provided, simply
pushes the values.

%closure-ref replaces references to free variables.
%stack-closure-ref
%heap-closure-ref

%vector-index used for referencing variables in closures and stack frames
Returns the index of NAME within the vector.

%heap-closure-set! used for closed-over-names in LETREC

%fetch-stack-closure returns a pointer to the current top of
 stack, which contains values (or cells for values) of the
 variables named in VECTOR.

Restrictions on Input:
----------------------
Special forms excluded: 
  ACCESS, DEFINE, DELAY, IN-PACKAGE, OR, SET!, THE-ENVIRONMENT
  UNASSIGNED?

Special forms introduced:
-------------------------
  none

Magic Cookies handled specially:
-------------------------------
 none

Guarantees on Output:
---------------------
 No non-local variable references.
  After LAMLIFT and CLOSCONV, the only free variables in lambda
expressions are those bound to static bindings (always available, e.g.
from the program counter).