Documentation for infix (cont), infix ∋ assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix (cont), infix ∋ §

multi sub infix:<(cont)>($a,$b --> Bool:D)

Membership operator.

Returns True if $a contains $b as an element.

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

Since release 2020.05, ∍ is an alias for this operator.

Language documentation: Sets, bags, and mixes §

From Sets, bags, and mixes

(Sets, bags, and mixes) infix (cont), infix ∋ §

Returns True if $a contains $b as an element, else False. More information, Wikipedia definition.