Exercise 2: A MaxDepth Visitor With Zero Node-Class Changes — Possible Solution
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THE NEW VISITOR
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    class MaxDepth:
        def visit_literal(self, node):
            return 1
        def visit_grouping(self, node):
            return 1 + node.expression.accept(self)
        def visit_unary(self, node):
            return 1 + node.right.accept(self)
        def visit_binary(self, node):
            return 1 + max(node.left.accept(self), node.right.accept(self))

A Literal is a leaf, so its own depth is 1. Every other node type's
depth is 1 (for itself) plus the deepest of whichever children it has
— exactly the recursive definition of tree depth, expressed once per
node type, with no isinstance check anywhere in the class.

RESULTS
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    MaxDepth of '2 + 3 * 4':           3
        Binary(+)                      depth 3
          Literal(2.0)                 depth 1
          Binary(*)                    depth 2
            Literal(3.0)               depth 1
            Literal(4.0)               depth 1

    MaxDepth of '((1 + 2) * (3 + 4))': 5
        Grouping                       depth 5
          Binary(*)                    depth 4
            Grouping                   depth 3
              Binary(+) -> two Literals   depth 2 / 1
            Grouping                   depth 3
              Binary(+) -> two Literals   depth 2 / 1

WHY THIS WORKS AS AN ANSWER
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This is the chapter's own central claim, made concrete a second time:
Literal, Grouping, Unary, and Binary were not opened, edited, or even
re-read to add this entirely new operation. MaxDepth is a genuinely
new capability over the exact same four node types NodeCounter already
used in the chapter — and, just like NodeCounter, it cost one new
class with one method per existing node type, zero edits anywhere
else. An isinstance-chain version of MaxDepth would have been a fourth
hand-written four-way type check, on top of the three the chapter
already counted (13 total isinstance(node, ...) checks across
evaluate_isinstance, stringify_isinstance, and count_nodes_isinstance)
— this exercise's MaxDepth would have pushed that real, growing total
to 17, while the Visitor-based accept() dispatch count stays at 4,
exactly where it's been since the first node class was written.