#`(
Sort of vague as to what we are trying to accomplish here. If we are just
trying to transform from one format to another, probably easiest to just
perform string manipulations.
)

my $level = '  ';

my $trees = q:to/END/;
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sub nested-to-indent { $^str.subst: / ^^ ($($level))* /, -> $/ { "{+$0} " }, :g }
sub indent-to-nested { $^str.subst: / ^^ (\d+) \s* /, -> $/ { "{$level x +$0}" }, :g }

say $trees;
say my $indent = $trees.&nested-to-indent;
say my $nest = $indent.&indent-to-nested;

use Test;
is($trees, $nest, 'Round-trip equals original');

#`(
If, on the other hand, we want perform more complex transformations; better to
load it into a native data structure which will then allow us to manipulate it
however we like.
)

# Import outline paragraph into native data structure
sub import (Str $trees, $level = '  ') {
    my $forest;
    my $last = -Inf;

    for $trees.lines -> $branch {
        $branch ~~ / ($($level))* /;
        my $this = +$0;
        $forest ~= do {
            given $this cmp $last {
                when More { "\['{esc $branch.trim}', " }
                when Same { "'{esc $branch.trim}', " }
                when Less { "{']' x $last - $this}, '{esc $branch.trim}', " }
            }
        }
        $last = $this;
    }

    sub esc { $^s.subst( /(<['\\]>)/, -> $/ { "\\$0" }, :g) }

    $forest ~= ']' x 1 + $last;
    $forest.EVAL;
}

my $forest = import $trees;

say "\nNative data structure:\n", $forest.raku;

{
    use JSON::Fast;
    say "\nJSON:\n", $forest.&to-json;
}

{
    use YAML;
    say "\nYAML:\n", $forest.&dump;
}