CS321 Prog Lang & Compilers                   Assignment # 6
Assigned: Feb. 5, 2007               Due: Wed. Feb. 14, 2007

Cut and paste the following into your solution file.
==================================================================

datatype RE
  = Empty
  | Union of RE * RE
  | Concat of RE * RE
  | Star of RE
  | C of char;

================================================================

The purpose of today's home work is to write functions analogous
to "first" and "follow" for context free grammars from todays
lecture. There are two differences, The functions you will write
for homework are for regular expressions, not context free
grammars, and since REs don't have non-terminal and terminal
symbols, the functions are for a complete RE rather than a
symbol.

Write 3 ML functions

1) Write   (nullable: RE -> boolean) Returns a boolean, true if
the empty string is a member of the set of strings recognized by
the RE, false otherwise.

2) Write  (first: RE -> char list) Returns a (char list) which
contains those characters which may appear as the first
character in the strings recognized by that RE.

3) Write  (last: RE -> char list). Returns a char list which
contains those characters which may appear as the last character
in the strings recognized by that RE.

All these functions a simple functions defined with pattern matching.
One clause for each constructor of RE.