Exercise 3: A Missing Closing Parenthesis — Possible Solution
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RESULT
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    parse("(2 + 3")

    SyntaxError correctly raised: expected ')' after expression

WHICH LINE CATCHES IT
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    def primary(self):
        ...
        if self.match('LPAREN'):
            expr = self.parse_expression()
            if not self.match('RPAREN'):
                raise SyntaxError("expected ')' after expression")   # <-- here
            return ('group', expr)

After consuming the opening '(' and successfully parsing the inner
expression "2 + 3", the parser tries to consume a closing ')' via
self.match('RPAREN'). Since the token stream has already reached EOF
at that point (there is no ')' left in the input), match() returns
None, the `if not self.match(...)` condition is true, and the explicit
SyntaxError fires with a specific, useful message.

WHY THIS WORKS AS AN ANSWER
------------------------------
This confirms the parser fails at the exact right moment - not
earlier (it doesn't reject "(2 + 3" prematurely while there's still a
valid expression to parse) and not later (it doesn't silently return a
successfully-parsed group node missing its own closing paren, which
would let a malformed program continue past error detection). This is
the parser-level equivalent of Chapter 1's own unterminated-string
check: both catch a genuinely unclosed construct at the specific point
where "there's nothing left to consume, and something was still
required" becomes knowable, rather than letting the problem surface
confusingly somewhere else - or not at all.