Documentation for infix ∉ assembled from the following types:

language documentation Sets, bags, and mixes

From Sets, bags, and mixes

(Sets, bags, and mixes) infix ∉

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

language documentation Operators

From Operators

(Operators) infix ∉

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␤» 

∉ is codepoint U+2209 (NOT AN ELEMENT OF).