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sub fibonacci (PositiveInt $n) {
    return 1 if $n <= 1;
    return fibonacci($n-1) + fibonacci($n-2);
}

sub fibonacci ($n ...
# Wait, wait... hold on a second! What is this $n thing you claim
...
    return 1 if $n <= 1;
    return fibonacci($n-1) + fibonacci($n-2);
}

sub fibonacci($n {$n >= 0 && int($n) == $n}) {
    return 1 if $n <= 1;
    return fibonacci($n-1) + fibonacci($n-2);
}

sub fibonacci($n) {
    die '$n failed check "$n >= 0 && int($n) == $n"' unless $n >= 0 &&
+ int($n) == $n;
    ...
}

sub fibonacci(PostiveInt $n) {

sub fibonacci($n {$n = PostiveInt->new($n)}) {