Lab Exercise 2

To allow for multiple expressions, you need to have some recursion.
For example, this rule for "exp" allows for arbitrarily long expressions:

exp : term | exp '+' term ;

So a single number (e.g. 1) is picked up by
	term
a following '+' and another number (e.g. 1 + 2) is picked up by
	exp '+' term
another following '+' and another number (e.g. 1 + 2 + 3) is picked up by
	exp '+' term
and so on.
I am assuming that there are other rules to recognise numbers as terms etc.


Similarly, one way to do this part of the lab is to change the rule for
"input" from:

input : exp ';'               { printf("\n"); printtree ($1, 1);
                                printf("=%d\n", evaluate ($1));
      ;

to:

input : exp ';'               { printf("\n"); printtree ($1, 1);
                                printf("=%d\n", evaluate ($1));
      | input exp ';'         { printf("\n"); printtree ($2, 1);
                                printf("=%d\n", evaluate ($2));
      ;

but there are several other equally good (if not better) ways.
Note that this allows for 1 or more expressions, which is not
exactly what is asked for. We can get 0 or more expressions by:

input :
      | input exp ';'         { printf("\n"); printtree ($2, 1);
                                printf("=%d\n", evaluate ($2));
      ;