Challenge 1: Adding a Clear Command — Possible Solution
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#[derive(clap::Subcommand)]
enum Commands {
    Add { description: String },
    List,
    Complete { id: u32 },
    Remove { id: u32 },
    Clear,
}

// Inside main()'s match cli.command block:
Commands::Clear => {
    tasks.clear();
}

WHY THIS WORKS AS AN ANSWER
------------------------------
Clear is added as a new variant of the Commands enum with NO
associated data — following the same pattern as the existing List
variant, which also carries no fields, since "clear everything" needs
no additional arguments the way Add's description or Complete/Remove's
id do.

Because clap::Subcommand is a DERIVE MACRO (this chapter's own
procedural-macro payoff from Chapter 4), simply adding the new Clear
variant to the enum is enough — clap automatically generates the
necessary CLI parsing logic to recognize a new "clear" subcommand with
no arguments, with no manual parser code to write by hand.

tasks.clear() is Vec's own built-in method for removing all elements
in place — the simplest possible way to empty the vector, matching
this chapter's existing Remove variant's use of Vec's built-in
.retain() method for a similar "modify the collection in place" style
of implementation. Adding this arm to the match block is required
because match, per Course 1 Chapter 6, is EXHAUSTIVE — the compiler
would refuse to compile main() with the new Clear variant present in
the enum but missing from this match block, correctly flagging that
every variant needs to be handled.