Exercise 1: Adding an IfNode — Possible Solution
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THE NEW NODE CLASS
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    class IfNode:
        def __init__(self, cond_name, body_nodes):
            self.cond_name = cond_name
            self.body_nodes = body_nodes
        def render(self, ctx):
            if ctx.get(self.cond_name):
                return ''.join(node.render(ctx) for node in self.body_nodes)
            return ''

This follows the chapter's own ForNode pattern exactly: it holds the
name of the thing it depends on (a condition name instead of a
collection name), plus its own already-parsed body_nodes. Nothing
about the body needs re-parsing at render time -- that work already
happened once, at compile time, exactly like ForNode's own body.

WIRING IT INTO THE COMPILER
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    def compile_if(template):
        m = re.search(r'\{%\s*if\s+(\w+)\s*%\}(.*?)\{%\s*endif\s*%\}', template, re.S)
        nodes = []
        if m:
            cond_name, body = m.groups()
            before, after = template[:m.start()], template[m.end():]
            nodes.extend(parse_to_nodes(before))
            nodes.append(IfNode(cond_name, parse_to_nodes(body)))
            nodes.extend(parse_to_nodes(after))
            return nodes
        return parse_to_nodes(template)

TESTING IT
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    if_template = "<p>{% if logged_in %}Welcome back, {{ user }}.{% endif %}</p>"
    tree = compile_if(if_template)   # parsed ONCE

    print(render_nodes(tree, {"logged_in": True, "user": "Sam"}))
    print(render_nodes(tree, {"logged_in": False, "user": "Sam"}))
    print(render_nodes(tree, {"user": "Sam"}))   # no "logged_in" key at all

RESULTS
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    Truthy flag:  <p>Welcome back, Sam.</p>
    Falsy flag:   <p></p>
    Missing flag: <p></p>

The truthy case renders the body in full, with its own VarNode
correctly substituting "Sam". Both the explicit False case and the
case where "logged_in" isn't in the context at all render an empty
body -- ctx.get(self.cond_name) returns None for a missing key, and
None is just as falsy as False, so no special "key doesn't exist"
handling was needed.

WHY THIS WORKS AS AN ANSWER
----------------------------
It adds a real node class following the chapter's own established
Node-with-a-render-method shape, wires it into the same one-time
compile step ForNode already uses (no re-parsing the body at render
time), and verifies all three real cases a boolean flag can actually
take -- true, explicitly false, and simply absent -- rather than only
checking the happy path.