Documentation for infix ∌ assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix ∌ §

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␤» 

Language documentation: Sets, bags, and mixes §

From Sets, bags, and mixes

(Sets, bags, and mixes) infix ∌ §

Returns True if $a does not contain $b, else False. More information, Wikipedia definition.