# Pastebin CGDZ7gJw
use nqp;
sub postcircumfix:<{; }>(\SELF, @indices, :$exists!) {
    multi recurse-at-key(\SELF, \indices, 0) {
		(False xx indices.map(*.elems).reduce(&infix:<*>)).List
	}
    multi recurse-at-key(\SELF, [@idx, +@indices]) {
		@idx.map( -> $idx { |recurse-at-key(SELF, ($idx, |@indices)) }).List
	}
	multi recurse-at-key(\SELF, [$idx, +@indices where *.elems > 0]) {
		nqp::if(
			nqp::not_i(SELF.EXISTS-KEY($idx)),
			recurse-at-key(SELF{$idx}, @indices, 0),
			recurse-at-key(SELF{$idx}, @indices),
		)
	}
	multi recurse-at-key(\SELF, [$idx]) {
		SELF.EXISTS-KEY($idx)
	}

	recurse-at-key SELF, @indices
}

my %a = 1 => {2 => {3 => 42}};

dd %a{1;2;3}:exists;
dd %a{-99;2;3}:exists;
dd %a{-99;2,3;4}:exists;
dd %a{1,1;2;3}:exists;
dd %a{1;2,2;3}:exists;
dd %a{1;2,2;3,3}:exists;
dd %a{1,1;2,2;3,3}:exists;
dd %a{1,1;-99,2;3,3}:exists;