Exercise 1: A Real {% else %} Branch — Possible Solution
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THE EXTENDED IfNode AND parse()
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    class IfNode:
        def __init__(self, cond_name, if_body, else_body=None):
            self.cond_name = cond_name
            self.if_body = if_body
            self.else_body = else_body or []

        def render(self, ctx):
            body = self.if_body if ctx.get(self.cond_name) else self.else_body
            return ''.join(node.render(ctx) for node in body)

parse() now has to know WHICH terminator it stopped on -- 'endif',
'endfor', or 'else' -- so an if-block can tell whether to keep parsing
a second body for the else branch. It returns a 3-tuple instead of 2:

    def parse(tokens, index=0):
        nodes = []
        while index < len(tokens):
            token = tokens[index]
            if token.startswith('{{'):
                nodes.append(VarNode(token[2:-2].strip()))
                index += 1
            elif token.startswith('{%'):
                tag = token[2:-2].strip()
                if tag.startswith('if '):
                    cond_name = tag[3:].strip()
                    if_body, index, term = parse(tokens, index + 1)
                    else_body = []
                    if term == 'else':
                        else_body, index, term = parse(tokens, index)
                    nodes.append(IfNode(cond_name, if_body, else_body))
                elif tag.startswith('for '):
                    m = re.match(r'for (\w+) in (\w+)', tag)
                    var, coll_name = m.group(1), m.group(2)
                    body, index, term = parse(tokens, index + 1)
                    nodes.append(ForNode(var, coll_name, body))
                elif tag in ('endif', 'endfor'):
                    return nodes, index + 1, tag
                elif tag == 'else':
                    return nodes, index + 1, 'else'
                else:
                    index += 1
            else:
                nodes.append(TextNode(token))
                index += 1
        return nodes, index, None

    def compile_template(source):
        nodes, _, _ = parse(tokenize(source))
        return nodes

VERIFIED, REAL RESULT
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    template = '{% if logged_in %}Welcome back!{% else %}Please log in.{% endif %}'
    nodes = compile_template(template)

    render_nodes(nodes, {'logged_in': True})   -> 'Welcome back!'
    render_nodes(nodes, {'logged_in': False})  -> 'Please log in.'

Nested inside a for loop, to confirm else-tracking doesn't get
confused by a sibling loop's own endfor:

    mixed = '{% for user in users %}{% if user %}<li>{{ user }}</li>{% else %}<li>(anonymous)</li>{% endif %}{% endfor %}'
    render_nodes(compile_template(mixed), {'users': ['Ada', '', 'Grace']})
    -> '<li>Ada</li><li>(anonymous)</li><li>Grace</li>'

WHY THIS WORKS AS AN ANSWER
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It follows the chapter's own IfNode/ForNode/parse() shape exactly,
adding only what's genuinely new: a way for parse() to report which
tag it stopped at, since an if-block now has two real possible
endings (an else it needs to keep parsing past, or an endif it
doesn't). Verifying the nested if-inside-for case confirms the fix
doesn't accidentally consume a sibling ForNode's own endfor as part
of the if-block's else-tracking.