Exercise 1: A Ternary (Conditional) Expression Node — Possible Solution
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THE NEW NODE TYPE
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    @dataclass
    class Ternary(Expr):
        cond: Expr
        then_branch: Expr
        else_branch: Expr
        def accept(self, visitor):
            return visitor.visit_ternary(self)

Same shape as every other node class in the chapter: one dataclass,
one accept() method that calls back into whichever visitor is walking
the tree. Nothing about adding a new node type is special-cased.

THE TWO VISIT METHODS
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    class Evaluator:
        ...
        def visit_ternary(self, node):
            return node.then_branch.accept(self) if node.cond.accept(self) else node.else_branch.accept(self)

    class AstPrinter:
        ...
        def visit_ternary(self, node):
            return f"(ternary {node.cond.accept(self)} {node.then_branch.accept(self)} {node.else_branch.accept(self)})"

RESULTS
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    Tree for '1 ? 2 : 3':  Ternary(Literal(1.0), Literal(2.0), Literal(3.0))
    tree.accept(Evaluator())  -> 2.0
    tree.accept(AstPrinter()) -> (ternary 1.0 2.0 3.0)

WHY THIS WORKS AS AN ANSWER
------------------------------
This is exactly the "adding an operation is cheap, adding a node type
is not" tradeoff the chapter names directly. Because Ternary is a NEW
node type, both existing visitors (Evaluator and AstPrinter) genuinely
needed a new method each — that cost is real and the chapter doesn't
pretend otherwise. What the chapter's own dispatch-count finding
predicts, and what this exercise confirms, is that neither existing
visitor needed its OTHER methods touched at all: visit_literal,
visit_binary, visit_unary, and visit_grouping are exactly as they were
before Ternary existed. The isinstance-chain equivalent would have
required inserting a new elif branch into the middle of every existing
function's own type-check chain instead of appending one clean new
method to the end of each class.