From type/Test

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multi sub isnt(Mu $got, Mu:U $expected, $desc = '')
multi sub isnt(Mu $got, Mu:D $expected, $desc = '')

Marks a test as passed if $got and $expected are not equal using the same rules as is(). The function accepts an optional description of the test.

isnt pi, 3, 'The constant π is not equal to 3';
my Int $a = 23;
$a = Nil;
isnt $a, Nil, 'Nil should not survive being put in a container';