CS321 Prog Lang & Compilers                  Assignment # 5
Assigned: Jan 29, 2007               Due: Wed. Jan 31, 2007

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1) Your job is to write a function that interprets regular expressions
as a set of strings.

- reToSetOfString;
val it = fn : RE -> string list

To do this you will need the definition of regular expressions (the
datatype RE) and the functions that implemenent sets of strings as
lists of strings without duplicates. Tou will also need the "cross"
operator from lecture 4. All these functionas can be found in the file
"assign5Prelude.html" which can be downloaded from the assignments page
of the course website. The first line of your solution should include
this file by using

use "assign5Prelude.html";

"reToSetOfString" is fairly easy to write (use pattern matching),
except some regular expressions represent an infinite set of strings.
These come from use of the Star operator. To avoid this we will write a
function that computes an approximate set of strings. Star will produce
0,1,2, and 3 repetitions only. For example:

reToSetOfString (Concat (C #"a",Star (C #"b"))) ---> ["abbb","abb","ab","a"]

BONUS 10 points. Write a version reToN which given an interger n
creates exactly 0,1, ... n  repetitions exactly.

reToN 2 (Concat (C #"a",Star (C #"b"))) ---> ["abb","ab","a"]
reToN 4 (Concat (C #"a",Star (C #"b"))) ---> ["abbbb","abbb","abb","ab","a"]