Challenge 1: A max_of_two! Macro — Possible Solution
====================================================================

macro_rules! max_of_two {
    ($a:expr, $b:expr) => {
        if $a > $b { $a } else { $b }
    };
}

fn main() {
    let result = max_of_two!(3, 7);
    println!("{}", result); // 7
}

WHY THIS WORKS AS AN ANSWER
------------------------------
The pattern ($a:expr, $b:expr) captures TWO separate expression
fragments, using two distinct metavariables — a direct extension of
this chapter's single-metavariable square! example to a two-argument
case, following the same $name:expr syntax for each.

The expansion — if $a > $b { $a } else { $b } — is ordinary Rust code
with $a and $b substituted in wherever they appear, exactly this
chapter's core definition of what a macro does: it produces Rust
syntax as output, which is then compiled normally as if it had been
written by hand at the call site.

Calling max_of_two!(3, 7) expands, at compile time, into
if 3 > 7 { 3 } else { 7 } — an ordinary if/else expression evaluating
to 7, which becomes result's value. Since $a and $b are expression
fragments (not just literals), this macro also works correctly with
more complex expressions passed in, like max_of_two!(x + 1, y * 2),
not just bare numeric literals.