From language/setbagmix

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Returns True if $a does not contain $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 does not contain $b. Equivalent to !(cont).

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