Documentation for infix ⊅ assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix ⊅ §

multi sub infix:<⊅>($a,$b --> Bool:D)

Not a superset of operator.

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

say (1,2,3) ⊅ (2,3,1); # OUTPUT: «True␤» 
say (1,2,3) ⊅ (2,3);   # 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 superset of $b, else False. More information, Wikipedia definition.