Documentation for infix ⊄ assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix ⊄ §

multi sub infix:<⊄>($a,$b --> Bool:D)

Not a subset of operator.

Returns True if $a is not a strict subset of $b. Equivalent to !(<).

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

Language documentation: Sets, bags, and mixes §

From Sets, bags, and mixes

(Sets, bags, and mixes) infix ⊄ §

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