From a starting seed and a bunch of operations, generate the closure

closure.pl

sub closure($elem, *@ops) {
    my $set = set $elem;
    my @queue = $elem;
    while @queue {
        my $e = pop @queue;
        for @ops.map(-> &op { &op($e)}) -> $d {
            next if $d (elem) $set;
            $set = $set (|) $d;
            push @queue, $d;
        }
    }
    $set;
}

sub swap($p1, $p2) {
    -> $s {
        my @s = $s.comb;
        @s[$p1, $p2].=reverse;
        @s.join
    }
}

say closure "abcd", swap(0, 1), swap(1, 2), swap(2, 3);