From language/setbagmix

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Returns True if $a is not an element of $b, else False. More information, Wikipedia definition.

From language/operators

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multi sub infix:<∉>($a,$b --> Bool:D)

Non-membership operator.

Returns True if $a is not an element of $b. Equivalent to !(elem).

say 4 ∉ (1, 2, 3);       # OUTPUT: «True␤» 
say 2 !(elem) (1, 2, 3); # OUTPUT: «False␤»