More on HW #3:

     - The assignment assumes input restricted to expressions involving
       well-formed grammar.  Error checking will on the most part not be
       implemented.  When there is error checking, it will return a list
       of token(s) containing the error message and will NOT print a
       message.


     - The DRAWING link shows the flow diagram for the parsing of the
       string "(3^4)^5".  Each parse_<nt> function takes a list of
       tokens, but for brevity in the diagram, the token list is
       represented by the string.  For instance, wherever there is
       "(3^4)^5" in the diagram, it is understood that this means 

       (list "(" "3" "^" "4" ")" "^" "5")

       and similarly, a "4)^5" represents

       (list "4" ")" "^" "5").

       Note also that in the homework, the supplied lexical analysis
       function, string->tokenlist is responsible for the initial
       breakup into tokens.  That is, the initial parse_fmla call should
       be 

       (parse_fmla (string->tokenlist "(3^4)^5")).

     - There are some conventions used in the drawing.  First, flow
       always go from top to bottom whenever possible, though each line
       should only be followed once.  Similarly, when two down lines are
       possible, the left one should be done first, and when the flow
       returns, the right one should be done.  A "notice <terminal>"
       implies that the function above it recognizes the <terminal>,
       uses this recognition to indicate what block of a conditional
       structure it should be in, and removes the <terminal> from the token
       list by passing rest (i.e. everything but the <terminal>) onto
       the next function in the flow.