'exists_fn' => sub {
	my $lhs = get_next();

	my %set;
	@set{ map { $id->($_) } @$lhs } = @$lhs;

	for (get_rand(), get_rand(), get_rand(), get_rand()) {
		my @int = grep { exists $set{$id->($_)} } @$_;
		return unless @int;
		undef %set;
		@set{ map { $id->($_) } @int } = @int;
	}
	return keys %set;
},

'slice_defined_fn' => sub {
	my $lhs = get_next();

	my %set;
	@set{ map { $id->($_) } @$lhs } = @$lhs;

	for (get_rand(), get_rand(), get_rand(), get_rand()) {
		my @int = grep { defined } @set{ map { $id->($_) } @$_ };
		return unless @int;
		undef %set;
		@set{ map { $id->($_) } @int } = @int;
	}
	return keys %set;
},

});