CS G262 Assignment #4.
Assigned:  Thursday, 30 September 2004
Due:       Thursday, 7 October 2004, by 6pm

Implement an attribute evaluator for quirk21 as specified below.
This phase shall define a top-level procedure named evaluator1
that takes an abstract syntax tree for a complete quirk21 program
and returns that tree, after recording certain E.dleaf, E.free,
and E.tail attributes via side effect as follows:

    For each lambda expression E with body E0, the evaluator1
    procedure uses astUpdate1 on E0 (not E) to record E0.dleaf
    using the encoding described by the table below:

        value    encoding
        -----    --------
        true         1
        false        0

    For every expression E, the evaluator1 procedure uses
    astUpdateFree to record E.free, represented as a list
    of Qsymbol, without duplicates.

    For each procedure call expression E, the evaluator
    operation uses astUpdate2 to record E.tail using the
    encoding described by the table above.

The evaluator1 procedure should have no side effects apart
from those listed above.

The instructor is providing Scheme code to parse a quirk21
program and to return an alpha-converted form of the program.
This code will be made available in

    /course/csg262/Assignments/Assignment4

That directory will include an implementation of the set
operations from SRFI-1.  Students are encouraged to use the
SRFI-1 operations instead of writing their own.  Students are
not allowed to use implementation-specific set operations, as
that would not be portable.  All submitted code should run in
any R5RS-conforming implementation of Scheme.

Please do not modify the instructor's code, as that would
make it harder to combine your attribute evaluator phase with
a code generator written by someone else.  If you find a bug
in the instructor's code, please report it.

Submit this assignment using the submit script that is
documented on the course web page.