CSC444 - Lea
                              Exercise 


Write a recursive descent parser to parse and evaluate boolean
expressions using the following grammar:

S ->        Assign EOL
    |       Query  EOL
Assign ->   ID EQ Exp
Query ->    ID QMARK
Exp ->      Exp AND Exp 
    |       Exp OR  Exp
    |       NOT Exp 
    |       LP Exp RP
    |       ID
    |       TRUE
    |       FALSE

Use a very simple yylex function to return the required terminals:

    a-z   &    |    ~     =    ?      0      1     (    )   \n
    ID    AND  OR   NOT   EQ   QMARK  FALSE  TRUE  LP   RP  EOL

Lower case letters are variables, initialized to FALSE. (You can
make an array of 26 elements and index into them by letter.)

The usual precedence and associativity rules for AND and OR apply.

The parse should restart after every newline. 
Report syntax errors by listing expected tokens.

Show the full LL-form grammar as part of your submission.

The program should behave something like this ("#" precedes user input):

# a = 1
# a?
  1
# b = a | 0
# c = b & ~b & (a | a)
# c?
  0
# ^C
...