Explanation of `expr'
.....................

   The `exp' grouping has several rules, one for each kind of
expression.  The first rule handles the simplest expressions: those
that are just numbers.  The second handles an addition-expression,
which looks like two expressions followed by a plus-sign.  The third
handles subtraction, and so on.

     exp:      NUM
             | exp exp '+'     { $$ = $1 + $2;    }
             | exp exp '-'     { $$ = $1 - $2;    }
             ...
             ;

   We have used `|' to join all the rules for `exp', but we could
equally well have written them separately:

     exp:      NUM ;
     exp:      exp exp '+'     { $$ = $1 + $2;    } ;
     exp:      exp exp '-'     { $$ = $1 - $2;    } ;
             ...

   Most of the rules have actions that compute the value of the
expression in terms of the value of its parts.  For example, in the
rule for addition, `$1' refers to the first component `exp' and `$2'
refers to the second one.  The third component, `'+'', has no meaningful
associated semantic value, but if it had one you could refer to it as
`$3'.  When `yyparse' recognizes a sum expression using this rule, the
sum of the two subexpressions' values is produced as the value of the
entire expression.  See Actions.

   You don't have to give an action for every rule.  When a rule has no
action, Bison by default copies the value of `$1' into `$$'.  This is
what happens in the first rule (the one that uses `NUM').

   The formatting shown here is the recommended convention, but Bison
does not require it.  You can add or change whitespace as much as you
wish.  For example, this:

     exp   : NUM | exp exp '+' {$$ = $1 + $2; } | ...

means the same thing as this:

     exp:      NUM
             | exp exp '+'    { $$ = $1 + $2; }
             | ...

The latter, however, is much more readable.